Most soundcards these days are capable of more than plain stereo output. To take advantage of this, as well as the speaker assignment flags, BASS has support for multi-channel user streams, and also has built-in support for multi-channel OGG, WAV and AIFF files. Add-ons provide support for other multi-channel formats.
When a stream having more channels than there are speakers is played, the extra channels will not be heard. The chans member of the BASS_CHANNELINFO structure can be used to check how many channels a stream has, and the speakers member of the BASS_INFO structure can be used to check how many speakers are available.