Have u noticed the new cool feature in Vista? It allows u to independently control the sound level for each application that uses sound. For example, say u want to keep Windows system sounds at a mid range level, and Media Player at a lower level. U can change the volume level settings by opening the speaker volume application (lower/right hand side of notification area of task bar), click mixer, and individually change the volume level for any application that uses sound and is currently running. U can even mute the sound for each individual application.
Happy computing!