the SDL_AudioStream to query.
On output, set to number of channels in the map. Can be NULL.
an array of the current channel mapping, with as many elements as the current output spec's channels, or NULL if default. This should be freed with SDL_free() when it is no longer needed.
Threadsafety: It is safe to call this function from any thread, as it holds a stream-specific mutex while running.
Get the current input channel map of an audio stream.
Channel maps are optional; most things do not need them, instead passing data in the order that SDL expects.
Audio streams default to no remapping applied. This is represented by returning NULL, and does not signify an error.