Most audio commands can operate on multiple channels.  The exceptions are
adcmd_waitcycle, cmd_write and cmd_read, which can only operate on one
channel at a time.  You specify the channel(s) that you want to use by
setting the appropriate bits in the ioa_request.io_unit field of the
IOAudio block.  If you send a command for a channel that you do not own,
your command will be ignored.  For more details, see the section on
"allocation and arbitration" below.