Added mixer and sound (c side)

src/mixer.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <stdint.h>
+#include "sound.h"
+
+
+void mixer_init(void);
+void mixer_deinit(void);
+int16_t const* mixer_getNextBlock(void);
+void mixer_play(sound_t const *sound);
+void mixer_mix(void);