A:

You can't, at least not meaningfully, and not on a cross-platform basis. Hardware providers like DirectSound3D/EAX won't accept stereo data at all, and the software mixer will route stereo channels to pairs of speakers which are adjacent in an arbitrarily-ordered list defined by Microsoft as part of the WDM driver model, which isn't a particularly-meaningful thing to do under most conditions.

You can call AIL_process_digital_audio to convert stereo data to mono where necessary. In general, you should just not use stereo sound data, though. It's almost certainly not what you want.