If you want the onboard audio device to become the primary audio device, remove the USB audio devices. The /dev/audio link will then point to the /dev/sound/0 entry. If the /dev/sound/0 entry is not the primary audio device, then either shutdown 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 links.