I am assuming that you have windows 7 or maybe vista.
if it is windows 7, windows 7 has a really annoying sound stack. I suspect that it is just a setting, you can avoid having to reinstall sound drivers this time. And any other time this might happen.
go to control panel ->sound->playback devices.
you should see a list of all playback devices make sure that your "speakers and headphones" says default sound device (not default communication device) if not right click on speakers and headphones, set as default device. this should fix your problem. my best bet is that skype may have reconfigured these settings. My sig. other had this problem when skype was installed.