A The Power Macintosh 5200/5300/6200/6300 sound input drivers do not support the
siActiveChannels selector. You will have to record in stereo and throw out the
channel that you did not want. The best place to do this is in the interrupt
routine: copy out the data only from the channel that you actually want to
record from.
In general, if a selector is not supported, SPBGetDeviceInfo and SPBSetDeviceInfo will return siUnknownInfoType. You will have to determine for yourself if this is a significant error as far as your project is concerned.