Volume Mixer in Windows 7, not just show device volume
but also lists all applications that are currently running
and producing the audio. You can control the audio of any
of the application from there on. So when you are having
more than one application that is producing sound you can
control from here and stop it as well.
At the time when i snipped the screen, there were few applications
running on my system, Yahoo Messenger, Google Talk, few media players
Windows Media Player, VLC Media player, QuickTime and one video clip
was also being played in Windows Live photo gallery (yes it can play
Video as well apart from Images.