audiomixer: Fix garbled playback when voice 0 is stopped, audio pops

There were two main problems
 - word_buffer was being filled as though with unsigned samples,
   but during mixing all samples are kept in signed mode
 - If the first buffer was stopped, the voices_active flag got set
   anyway, even though the output buffer wasn't initialized yet,
   so the samples were mixed with indeterminate data

We also cover the case where no buffer was playing, and ensure
the output buffer is filled.

This now works much better.  Tested on neotrellis m4 playing back
4 mp3 streams at a time in signed-16, 22050Hz

Author: jepler

shared-module/audiomixer/Mixer.c

@@ -143,19 +143,17 @@ static inline uint32_t pack8(uint32_t val) {
 static void mix_down_one_voice(audiomixer_mixer_obj_t* self,
         audiomixer_mixervoice_obj_t* voice, bool voices_active,
         uint32_t* word_buffer, uint32_t length) {
-    bool voice_done = voice->sample == NULL;
-    while (!voice_done && length != 0) {
+    while (length != 0) {
         if (voice->buffer_length == 0) {
             if (!voice->more_data) {
                 if (voice->loop) {
                     audiosample_reset_buffer(voice->sample, false, 0);
                 } else {
                     voice->sample = NULL;
-                    voice_done = true;
                     break;
                 }
             }
-            if (!voice_done) {
+            if (voice->sample) {
                 // Load another buffer
                 audioio_get_buffer_result_t result = audiosample_get_buffer(voice->sample, false, 0, (uint8_t**) &voice->remaining_buffer, &voice->buffer_length);
                 // Track length in terms of words.
@@ -230,10 +228,8 @@ static void mix_down_one_voice(audiomixer_mixer_obj_t* self,
     }
 
     if (length && !voices_active) {
-        uint32_t sample_value = self->bits_per_sample == 8
-            ?  0x80808080 : 0x80008000;
         for (uint32_t i = 0; i<length; i++) {
-            word_buffer[i] = sample_value;
+            word_buffer[i] = 0;
         }
     }
 }
@@ -269,9 +265,16 @@ audioio_get_buffer_result_t audiomixer_mixer_get_buffer(audiomixer_mixer_obj_t*
 
         for (int32_t v = 0; v < self->voice_count; v++) {
             audiomixer_mixervoice_obj_t* voice = MP_OBJ_TO_PTR(self->voice[v]);
+            if(voice->sample) {
+                mix_down_one_voice(self, voice, voices_active, word_buffer, length);
+                voices_active = true;
+            }
+        }
 
-            mix_down_one_voice(self, voice, voices_active, word_buffer, length);
-            voices_active = true;
+        if (!voices_active) {
+            for (uint32_t i = 0; i<length; i++) {
+                word_buffer[i] = 0;
+            }
         }
 
         if (!self->samples_signed) {