Here is the situation:
While watching a youtube video, sometimes, the sound will stop working (even the beggining of the video, which I saw WITH sound, no longer plays the sound) but the image still works (video keeps playing) Simply restarting firefox solves the issue..
update: same problem occurs in epiphany (another web browser available for ubuntu) Restarting epiphany also solves the problem..
fyi: I use Firefox 3.5 on Ubuntu 9.10 Karmic koala
If there is anything more you need to help me out, I'll edit it in this post.
Thanks in advance!