This is a callback that signifies that a channel has finished all its loops and has completed playback.
Defined in <SDL3_mixer/SDL_mixer.h>
typedef void (SDLCALL *Mix_EffectDone_t)(int chan, void *udata);
This gets called if the buffer plays out normally, or if you call Mix_HaltChannel(), implicitly stop a channel via Mix_AllocateChannels(), or unregister a callback while it's still playing.
DO NOT EVER call SDL_LockAudio() from your callback function!