Apple O/S system updates often cause this problem. One of the many reasons I was happy to get the Mac out of my system.
Try disconnecting the cable, rebooting both devices and then reconnecting the cable. Go back to Sound and see if it reappears in the list. If that doesn't work, call Apple support because it could be a hidden file that got corrupted or something similar that you'd never find on your own. Worst case you will have to reinstall your O/S. Frustrating stuff! Cheers,
Spencer
Try disconnecting the cable, rebooting both devices and then reconnecting the cable. Go back to Sound and see if it reappears in the list. If that doesn't work, call Apple support because it could be a hidden file that got corrupted or something similar that you'd never find on your own. Worst case you will have to reinstall your O/S. Frustrating stuff! Cheers,
Spencer