3. Conflicts

When using SDL_mixer functions you need to avoid the following functions from SDL:

SDL_OpenAudio

Use Mix_OpenAudio instead.

SDL_CloseAudio

Use Mix_CloseAudio instead.

SDL_PauseAudio

Use Mix_Pause(-1) and Mix_PauseMusic instead, to pause.
Use Mix_Resume(-1) and Mix_ResumeMusic instead, to unpause.

SDL_LockAudio

This is just not needed since SDL_mixer handles this for you.
Using it may cause problems as well.

SDL_UnlockAudio

This is just not needed since SDL_mixer handles this for you.
Using it may cause problems as well.

You may call the following functions freely:

SDL_AudioDriverName

This will still work as usual.

SDL_GetAudioStatus

This will still work, though it will likely return SDL_AUDIO_PLAYING even though SDL_mixer is just playing silence.

It is also a BAD idea to call SDL_mixer and SDL audio functions from a callback. Callbacks include Effects functions and other SDL_mixer audio hooks.