If you want to perform multi-channel commands, all the channels must have
the same key since the IOAudio block has only one allocation key field.
The channels must all have that same key even when they were not allocated
simultaneously. If you want to use a key you already have, you can pass
that key in the ioa_AllocKey field and ADCMD_ALLOCATE can allocate other
channels with that existing key. The ADCMD_ALLOCATE command returns a new
and unique key only if you pass it a zero in the allocation key field.