Volume Mixer is one of those little known hidden features of Windows Vista for sound management. Volume Mixer gives each running multimedia program or application its own sound volume, independant of the overall sound level of your system. This is unlike Windows XP wherein a change in the volume slider in the system tray affects the overall sound of your computer.

To use Volume Mixer on Windows Vista, click on the Volume icon in the system tray and select Mixer. This will display a list of applications and programs having multimedia capabilities.
To increase or decrease the sound for an application move the level up or down. To mute/unmute an application, just click on the speaker button beneath the slider.