My system (running Karmic) has been quite stable, but a week ago, it decided to
stop outputting internet audio. Youtube, DtownTV, streaming audio from my radio
station -- nothing works. Mythtv audio works fine, but Rythmbox doesn't produce
any sound from a CD. (This may be a different problem.)

I have tried Youtube with Seamonkey, Opera, and Chrome. I even tried rebooting.

Any ideas?