No Sound - Please help!

Hello, I've been recently experiencing difficulties with sound on my Dell Inspiron Laptop. I am using Vista.

The problem is, when I try to play audio tracks on Winamp, Itunes, or Windows Media Player, the file won't play. It will just stay at 0:00. When I play videos, I can watch the video but their will be no sound. When I play downloaded games, there is no sound.

But if I go to a website like youtube, imeem, myspace, or play games online, there is sound.

Does anyone have any solutions for me? I would greatly appreciate it. It might be from a windows update, but I have uninstalled the most recent ones and even did a system restore hoping to fix the problem. Please help! Thanks!