Problem

You are listening to music and receive a skype call. You pause the music, take the call and after you hang up you restart your music only to find that the volume has been turned way down and only closing the app (be it Winamp, Windows Media Player, Media Player Classic, etc,) and restarting it can the volume be restored.

Solution

Control Panel > Sound > Communications > Select "Do Nothing"

By default, Windows 7 will automatically reduce the volume of other applications by 80% when a skype (or other) call is detected, but it often fails to restore the volume when the call is completed.