Select one of the following to change the primary USB audio device.
If you want the on-board audio device to become the primary audio device, remove the USB audio devices. The /dev/audio link then points to the /dev/sound/0 entry. If the /dev/sound/0 entry is not the primary audio device, then either shut down the system and use the boot -r command, or run the devfsadm -i command as root.
If you want the USB audio device to become primary audio device, just plug it in and check the device links.