Laptop Sound Does Not Work

Hello,

Hey, I have a problem. Sometimes when I start my Dell Studio 17 Laptop (Running Windows 7), the start up sound doesn't play. If it doesn't, when I log into my account, no sound, videos won't play (the whole player will freeze up), and media players freeze up. Even weirder, AIM freezes up too. What's wrong with my laptop, and how do I fix it?