How to play sound as default to all 8 channels of a CMI8768 based card.
Put this in /etc/asound.conf:

# Default output
pcm.!default {
    type plug
    # Use default surround71 pcm of card #1 (second card) 
    slave.pcm "surround71:1"
    slave.channels 8
    route_policy duplicate
}

# Manage volume with the hw mixer of card #1
ctl.!default {
    type hw
    card 1
}


Don't forget to adjust your card number, here the cmi is the second one (sis
integrated on the mainboard is the first).