Ok, yesterday the sound worked great, both the internal speakers and my headphones. But today it doesn't work at all. I cant find out what the problem is, I tried with rebooting the system and log in and out. My sound card is a ATI sound card, and the speakers are RV710 i think. I would be really glad if I could get some help. If it matters i use Ubuntu 11.10
As you happen to know your Hardware why don't you give this a try?
Click on the volume icon and go to sound preferences. Under the hardware tab, select the correct hardware in case it has changed.
Under the output tab select the same thing you selected in the hardware tab.
Lastly see if anything is muted. If so un-mute it. :)