port: begin implementing audio

mixer taken from sm64-port and modified to fit PD's naudio thing

MP3 playback not supported yet

audio buffering sucks

include/PR/abi.h

@@ -265,6 +265,8 @@ typedef short RESAMPLE_STATE[16];
  */
 typedef short ENVMIX_STATE[40];
 
+#ifdef PLATFORM_N64
+
 /*
  * Macros to assemble the audio command list
  */
@@ -402,6 +404,12 @@ typedef short ENVMIX_STATE[40];
         _a->words.w1 = (unsigned int) d;                                \
 }
 
+#else // PLATFORM_N64
+
+#include "mixer.h"
+
+#endif
+
 #endif /* _LANGUAGE_C */
 
 #endif /* !_ABI_H_ */