Steve Ball, Principal Program Manager Lead on the Windows Sound Team has given some background on sound schemes in Windows as well as the new sound schemes released as Ultimate Extras and their relationship with the default Windows Vista scheme.
Default Windows Vista Sound Scheme: The default Windows Vista sound scheme was designed with the same principles that were used in designing the Windows Vista visual elements and desktop experience. In contrast, the Windows XP sounds, while appropriate at the time and for that product design, were very ‘Western' and literal, using pianos and western orchestral instruments. The XP sounds were designed to complement the ‘photo-realistic' Bliss desktop (blue sky, green grass photo.) The Windows XP sounds can also be rather percussive and jarring in the context of day to day PC use, so it was an explicit goal to re-orchestrate the default Windows Vista sounds to complement the softer, cleaner theme and user interface elements in Windows Vista.
For Windows Vista, it was an intentional design goal to avoid ‘reinventing' the User Interface language for sound. For example, the "new mail" sound in Windows XP and in Windows Vista consist of the same pitches, interval, and timing.
New Mail (Notify): The Windows Vista ‘new mail' sound has simply been re-orchestrated to match the softer, more -rounded Windows Vista Startup Sound whose ‘sonic palette' was derived from the gentle and flowing Robert Fripp Soundscapes sessions that were recorded at Microsoft Studios in 2005 and 2006.