I stumbled on a little bug today while trying to use my Sennheiser USB headset adapter with my Sennheiser headphones. I plugged them in and got nothing...
The device would appear in System - Preferences - Sound, but on the available output tab it didn't show.
A bit more probing found that Alsa worked just fine with it.
Finally I determined that Pulseaudio wouldn't fully recognize the headset after it was plugged in until it was restarted. To do this you can either log out and log back in or type in the following commands at a console:
pulseaudio -k pulseaudio --check
Be sure you don't have any software open that's using the speakers or mic at the time or that program will likely freeze up.