Another interesting idea would be to improve multichannel support in our
   sound system in general.  It should be possibly not only to support sound
   cards that offer more than two channels, but also to bond channel providers
   together into composite devices -- so, for instance, I could use three
   Griffin iMic USB audio widgets to do 5.1 output to my amplifier.  Just a
   thought; the mention of ALSA reminded me that there are a number of small
   ways in which our sound subsystem could use some work...


i don't see why our audio subsystem doesn't already support 5.1 devices.

the merging thing would need work, but as far as i can tell, we support
a variable number of channels.  from <sys/audioio.h>:

        u_int   channels;       /* number of channels, usually 1 or 2 */