This is the format of a special effect callback:
Defined in <SDL3_mixer/SDL_mixer.h>
typedef void (SDLCALL *Mix_EffectFunc_t)(int chan, void *stream, int len, void *udata);myeffect(int chan, void *stream, int len, void *udata);
(chan) is the channel number that your effect is affecting. (stream) is the buffer of data to work upon. (len) is the size of (stream), and (udata) is a user-defined bit of data, which you pass as the last arg of Mix_RegisterEffect(), and is passed back unmolested to your callback. Your effect changes the contents of (stream) based on whatever parameters are significant, or just leaves it be, if you prefer. You can do whatever you like to the buffer, though, and it will continue in its changed state down the mixing pipeline, through any other effect functions, then finally to be mixed with the rest of the channels and music for the final output stream.
DO NOT EVER call SDL_LockAudio() from your callback function!