Get the number of converted/resampled bytes available.
Defined in SDL_audio.h
int SDL_AudioStreamAvailable(SDL_AudioStream *stream);
The stream may be buffering data behind the scenes until it has enough to resample correctly, so this number might be lower than what you expect, or even be zero. Add more data or flush the stream if you need the data now.
This function is available since SDL 2.0.7.