SDL_MixAudio- Mix audio data
void SDL_MixAudio(Uint8 *dst, Uint8 *src, Uint32 len, int volume);
This function takes two audio buffers of len bytes each of the playing audio
format and mixes them, performing addition, volume adjustment, and overflow
clipping. The volume ranges from 0 to SDL_MIX_MAXVOLUME and should be set to the
maximum value for full audio volume. Note this does not change hardware volume.
This is provided for convenience -- you can mix your own audio data.