I'm running Ubuntu on a Toshiba C660 (4GB RAM, Intel Core i5). My soundboard is integrated and after trying to play a song on youtube my volume is muted. I can't unmute it (appears like it's unmuted , but it's mute)
I had a similar problem, I just recently upgrade from 11.10 to 12.04 . In my case i was able to hear sound on System -> Sound -> Test Sound. But I couldn't hear a sound on youtube or while playing mp3's on rhythmbox.
I followed instruction at https://help.ubuntu.com/community/SoundTroubleshootingProcedure solution for 12.04 And now everything is back to normal.
I hope it helps
Given your sparse information and my limited knowledge on the topic, I can only offer a possible solution from my own experience: pulseaudio and flash do seem to cause problems from time to time. Killing one or the other solves this problem often for me: e.g.