I don't get any sound on streaming audio or video. However, anything on my hard drive plays just fine: downloaded videos, mp3's, podcasts, everything. I also get all the system sounds.
I've checked everything that comes to mind. I usually use this computer on a wireless network, but I hooked up to the cable modem via ethernet just to check if it was to do with wireless. Same results: no streaming audio.
What could it be?