Wow that has got to suck. First thought that comes to mind is a virus, trojan, or worm. Try running your anti-virus software, and any spy ware and ad ware removal programs you might have.

If you don't have these you can search the forums(upper right between f.a.q. and home), you will find links to several free ones.

My second thought would be something has changed in your sound card settings.

It would help if you listed your computer specs, and operating system.