Many Longhorn event sounds (like Windows Exclamation.wav ) were designed with precise zero-crossing points for seamless looping. MP3 encoding adds silence padding, breaking the loop.
In 2004, Microsoft famously "reset" Longhorn development because the code had become an unmanageable mess. Much of the original work, including specific UI elements and their accompanying sounds, was scrapped or heavily modified for the eventual release of Windows Vista in 2007. windows longhorn sounds download wav
PowerShell play-at-login script: