mute.fm mutes your music while watching videos. For Windows 7 and Windows 8 Desktop.
You can run this or view the source in Visual Studio 2010 Express (downloadable for free from Microsoft).
Running/building mute.fm+ requires installing Awesomium. Running/building mute.fm or mute.fm+ requires installing Innosetup. Deploying requires placing pscp into C:\.
Get the software here: http://www.mutefm.com/
Email: mutefmapp at gmail.com
Note: I don't use Windows much anymore (building this was my last project in which I used it), so I am not great at updating this with bug fixes, feature requests, etc. but if you want to dig in the code I'll try to point you in the right direction.
Have a Mac? Check out Background Music.