What I really like in Vista and particularly in this build is that you define different sound settings for your applications. So if you are listening to a podcast or viewing a Webcast you don't want to have your Messenger sounds interfering with that.
Here's a screenshot of my settings I usually use:
However what I somehow miss is the is in the Mobile center where we can define presentation settings that we are not able to select the applications you don't want to have sound enabled for. The workaround for that is to set the settings onto the volume mixer for those items that you certainly do not want the sound to be enabled. Anyway it's a real improvement if you compare it with Windows XP.
As you may know we changed the audio stack quite a bit here is a whitepaper about the Audio Innovations in Vista.