Mix sound sources in 32-bit buffer, then clip to 16-bit for output. Fixes overflow in some situations, eg MegaRace on SB16.

src/sound_speaker.c

@@ -37,7 +37,7 @@ void speaker_update()
         }
 }
 
-static void speaker_get_buffer(int16_t *buffer, int len, void *p)
+static void speaker_get_buffer(int32_t *buffer, int len, void *p)
 {
         int c;