Thanks for your reply. I've no longer got Virtualbox installed as I said
on my first message. Removing it was one of the things I did to try to
get sound and video back. No luck.

I'm still on silence and poor graphics. I don't know what to do, so
suggestions would be greatly appreciated. Do I have to reinstall/update
the whole system and lose all my settings?

I forgot to say: I've got Ubuntu 8.04 (hardy) 64 bits.