I've got a 7 month old Dell Vostro laptop running Windows 7 with a Core I7 and an nVidia GTX 630M.
For a while now, the sound on my computer has been all jacked up. Sometimes I'll open a Youtube video in Chrome and i'll have to restart chrome or reload the web page multiple times for the sound to come through. (yes, i'm sure that the sound isn't muted.) Sometimes it'll play fine a few times and then i'll go to another application and the sound won't work anymore. I look in the windows sound mixer, and it shows chrome as being open, but doesn't display any sound coming through. Sometimes, the entire sound will cut out and no application will play sounds, not even when i'm adjusting the sound levels using the mixer. However, when I go to restart, the system plays the log off and shutdown sounds fine.
I've run dxdiag, no problems found. Not sure what is going on.