Long Zheng has a comparison of sounds between Windows XP and Windows Vista. Jim Allchin tells us "why".
Sounds are obviously an important part of how people interact with their PC. The idea is to use your ears to get information from a sound without even thinking about it. For example, certain sounds on our PC tell us when we have a new email or instant message.
It was our goal for the sounds to be just noticeable enough that if they weren’t there you would miss them, but they’re subtle enough that they don’t get in your way. Just like the visual cues of the new Start button, the audio cues of the new sounds are "rounded" and "translucent." The default sound scheme in Windows Vista is intentionally much gentler and softer than in previous versions.