diff --git a/src/libespeak-ng/compiledata.c b/src/libespeak-ng/compiledata.c
index 572961df..d3135ef3 100644
--- a/src/libespeak-ng/compiledata.c
+++ b/src/libespeak-ng/compiledata.c
@@ -37,8 +37,8 @@
 #include "error.h"
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "spect.h"
 
 typedef struct {
diff --git a/src/libespeak-ng/compiledict.c b/src/libespeak-ng/compiledict.c
index 509b8fa7..38a67ad4 100644
--- a/src/libespeak-ng/compiledict.c
+++ b/src/libespeak-ng/compiledict.c
@@ -35,6 +35,7 @@
 #include "error.h"
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 
diff --git a/src/libespeak-ng/compilembrola.c b/src/libespeak-ng/compilembrola.c
index 919821ee..2f96ba47 100644
--- a/src/libespeak-ng/compilembrola.c
+++ b/src/libespeak-ng/compilembrola.c
@@ -31,6 +31,7 @@
 #include "error.h"
 #include "phoneme.h"
 #include "speech.h"
+#include "voice.h"
 #include "synthesize.h"
 
 static const char *basename(const char *filename)
diff --git a/src/libespeak-ng/dictionary.c b/src/libespeak-ng/dictionary.c
index 4ce6d8df..5bcfdbe9 100644
--- a/src/libespeak-ng/dictionary.c
+++ b/src/libespeak-ng/dictionary.c
@@ -33,6 +33,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 
diff --git a/src/libespeak-ng/error.c b/src/libespeak-ng/error.c
index b83f2214..c8b629d6 100644
--- a/src/libespeak-ng/error.c
+++ b/src/libespeak-ng/error.c
@@ -29,6 +29,7 @@
 #include "error.h"
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 
 espeak_ng_STATUS
diff --git a/src/libespeak-ng/espeak_api.c b/src/libespeak-ng/espeak_api.c
index c50f4776..cf0e21af 100644
--- a/src/libespeak-ng/espeak_api.c
+++ b/src/libespeak-ng/espeak_api.c
@@ -29,6 +29,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 #include "event.h"
diff --git a/src/libespeak-ng/intonation.c b/src/libespeak-ng/intonation.c
index 7c61faaf..a505be48 100644
--- a/src/libespeak-ng/intonation.c
+++ b/src/libespeak-ng/intonation.c
@@ -30,8 +30,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 /* Note this module is mostly old code that needs to be rewritten to
diff --git a/src/libespeak-ng/klatt.c b/src/libespeak-ng/klatt.c
index d8fae5eb..6cd9bf21 100644
--- a/src/libespeak-ng/klatt.c
+++ b/src/libespeak-ng/klatt.c
@@ -37,8 +37,8 @@
 #include "speech.h"
 #include "klatt.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 
 extern unsigned char *out_ptr;
 extern unsigned char *out_start;
diff --git a/src/libespeak-ng/numbers.c b/src/libespeak-ng/numbers.c
index 015cba47..77868841 100644
--- a/src/libespeak-ng/numbers.c
+++ b/src/libespeak-ng/numbers.c
@@ -33,8 +33,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 #define M_LIGATURE  0x8000
diff --git a/src/libespeak-ng/phonemelist.c b/src/libespeak-ng/phonemelist.c
index 5339161e..ee5a7ce7 100644
--- a/src/libespeak-ng/phonemelist.c
+++ b/src/libespeak-ng/phonemelist.c
@@ -30,6 +30,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 
diff --git a/src/libespeak-ng/readclause.c b/src/libespeak-ng/readclause.c
index 39a3f026..1a78af7c 100644
--- a/src/libespeak-ng/readclause.c
+++ b/src/libespeak-ng/readclause.c
@@ -39,8 +39,8 @@
 #include "error.h"
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 #define N_XML_BUF   500
diff --git a/src/libespeak-ng/setlengths.c b/src/libespeak-ng/setlengths.c
index 241ab658..754660dd 100644
--- a/src/libespeak-ng/setlengths.c
+++ b/src/libespeak-ng/setlengths.c
@@ -30,11 +30,10 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
-extern int GetAmplitude(void);
 extern void DoSonicSpeed(int value);
 extern int saved_parameters[];
 
diff --git a/src/libespeak-ng/spect.c b/src/libespeak-ng/spect.c
index 4ea7160b..7bb0bb94 100644
--- a/src/libespeak-ng/spect.c
+++ b/src/libespeak-ng/spect.c
@@ -32,12 +32,11 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "spect.h"
 
 extern double ConvertFromIeeeExtended(unsigned char *bytes);
-extern int PeaksToHarmspect(wavegen_peaks_t *peaks, int pitch, int *htab, int control);
 
 extern unsigned char pk_shape1[];
 extern int pk_select;
diff --git a/src/libespeak-ng/speech.c b/src/libespeak-ng/speech.c
index c5685bbc..36465aff 100644
--- a/src/libespeak-ng/speech.c
+++ b/src/libespeak-ng/speech.c
@@ -50,8 +50,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 #include "espeak_command.h"
 #include "fifo.h"
diff --git a/src/libespeak-ng/synth_mbrola.c b/src/libespeak-ng/synth_mbrola.c
index f9bd3a3a..f6b93013 100644
--- a/src/libespeak-ng/synth_mbrola.c
+++ b/src/libespeak-ng/synth_mbrola.c
@@ -33,14 +33,13 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
-#include "voice.h"
 
 #ifdef INCLUDE_MBROLA
 
 extern int Read4Bytes(FILE *f);
-extern void SetPitch2(voice_t *voice, int pitch1, int pitch2, int *pitch_base, int *pitch_range);
 extern unsigned char *outbuf;
 
 #if defined(_WIN32) || defined(_WIN64)
diff --git a/src/libespeak-ng/synthdata.c b/src/libespeak-ng/synthdata.c
index 2d4f27a1..039235ac 100644
--- a/src/libespeak-ng/synthdata.c
+++ b/src/libespeak-ng/synthdata.c
@@ -34,8 +34,8 @@
 #include "error.h"
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 const char *version_string = PACKAGE_VERSION;
diff --git a/src/libespeak-ng/synthesize.c b/src/libespeak-ng/synthesize.c
index 48b14967..d504fc93 100644
--- a/src/libespeak-ng/synthesize.c
+++ b/src/libespeak-ng/synthesize.c
@@ -33,8 +33,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 extern FILE *f_log;
diff --git a/src/libespeak-ng/synthesize.h b/src/libespeak-ng/synthesize.h
index eb001848..008f9427 100644
--- a/src/libespeak-ng/synthesize.h
+++ b/src/libespeak-ng/synthesize.h
@@ -74,7 +74,6 @@ extern "C"
 extern int embedded_value[N_EMBEDDED_VALUES];
 extern int embedded_default[N_EMBEDDED_VALUES];
 
-#define N_PEAKS   9
 #define N_PEAKS2  9 // plus Notch and Fill (not yet implemented)
 #define N_MARKERS 8
 
@@ -453,14 +452,17 @@ extern int wcmdq_head;
 extern int wcmdq_tail;
 
 // from Wavegen file
-int  WcmdqFree();
-void WcmdqStop();
-int  WcmdqUsed();
-void WcmdqInc();
+int  WcmdqFree(void);
+void WcmdqStop(void);
+int  WcmdqUsed(void);
+void WcmdqInc(void);
 void WavegenInit(int rate, int wavemult_fact);
 float polint(float xa[], float ya[], int n, float x);
-int WavegenFill();
+int WavegenFill(void);
 void MarkerEvent(int type, unsigned int char_position, int value, int value2, unsigned char *out_ptr);
+int GetAmplitude(void);
+void SetPitch2(voice_t *voice, int pitch1, int pitch2, int *pitch_base, int *pitch_range);
+int PeaksToHarmspect(wavegen_peaks_t *peaks, int pitch, int *htab, int control);
 
 extern unsigned char *wavefile_data;
 extern int samplerate;
diff --git a/src/libespeak-ng/tr_languages.c b/src/libespeak-ng/tr_languages.c
index 6556d224..0cbbb04d 100644
--- a/src/libespeak-ng/tr_languages.c
+++ b/src/libespeak-ng/tr_languages.c
@@ -32,6 +32,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 
diff --git a/src/libespeak-ng/translate.c b/src/libespeak-ng/translate.c
index 7ddb49af..ca878fd0 100644
--- a/src/libespeak-ng/translate.c
+++ b/src/libespeak-ng/translate.c
@@ -33,8 +33,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 Translator *translator = NULL; // the main translator
diff --git a/src/libespeak-ng/voice.h b/src/libespeak-ng/voice.h
index 0765aaa7..ff7d7a79 100644
--- a/src/libespeak-ng/voice.h
+++ b/src/libespeak-ng/voice.h
@@ -22,6 +22,8 @@ extern "C"
 {
 #endif
 
+#define N_PEAKS   9
+
 typedef struct {
 	char v_name[40];
 	char language_name[20];
diff --git a/src/libespeak-ng/voices.c b/src/libespeak-ng/voices.c
index c7a1a75e..6aed8c25 100644
--- a/src/libespeak-ng/voices.c
+++ b/src/libespeak-ng/voices.c
@@ -40,8 +40,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 #include "translate.h"
 
 MNEM_TAB genders[] = {
diff --git a/src/libespeak-ng/wavegen.c b/src/libespeak-ng/wavegen.c
index dd656f9c..178fc6a1 100644
--- a/src/libespeak-ng/wavegen.c
+++ b/src/libespeak-ng/wavegen.c
@@ -33,8 +33,8 @@
 
 #include "speech.h"
 #include "phoneme.h"
-#include "synthesize.h"
 #include "voice.h"
+#include "synthesize.h"
 
 #if HAVE_SONIC_H
 #include "sonic.h"
@@ -665,7 +665,7 @@ static int ApplyBreath(void)
 	return value;
 }
 
-int Wavegen()
+static int Wavegen()
 {
 	if (wvoice == NULL)
 		return 0;
@@ -1117,7 +1117,7 @@ void SetPitch2(voice_t *voice, int pitch1, int pitch2, int *pitch_base, int *pit
 	*pitch_range = base + (pitch2 * range)/2 - *pitch_base;
 }
 
-void SetPitch(int length, unsigned char *env, int pitch1, int pitch2)
+static void SetPitch(int length, unsigned char *env, int pitch1, int pitch2)
 {
 	if (wvoice == NULL)
 		return;
@@ -1140,7 +1140,7 @@ void SetPitch(int length, unsigned char *env, int pitch1, int pitch2)
 	flutter_amp = wvoice->flutter;
 }
 
-void SetSynth(int length, int modn, frame_t *fr1, frame_t *fr2, voice_t *v)
+static void SetSynth(int length, int modn, frame_t *fr1, frame_t *fr2, voice_t *v)
 {
 	if (wvoice == NULL || v == NULL)
 		return;
@@ -1246,7 +1246,7 @@ void Write4Bytes(FILE *f, int value)
 	}
 }
 
-int WavegenFill2()
+static int WavegenFill2()
 {
 	// Pick up next wavegen commands from the queue
 	// return: 0  output buffer has been filled
diff --git a/tests/api.c b/tests/api.c
index 9a48859e..b017e6ec 100644
--- a/tests/api.c
+++ b/tests/api.c
@@ -28,6 +28,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"
 
diff --git a/tests/readclause.c b/tests/readclause.c
index 2692ba10..8b465118 100644
--- a/tests/readclause.c
+++ b/tests/readclause.c
@@ -31,6 +31,7 @@
 
 #include "speech.h"
 #include "phoneme.h"
+#include "voice.h"
 #include "synthesize.h"
 #include "translate.h"