Longhorn (2003–2004 builds, e.g., 3683, 4015, 4074, 4093) used a unique, ethereal sound set distinct from final Windows XP or Vista. These sounds are often sought for retro theme builds.
Most operating system sounds are functional—they alert you to an error or a new email. But the Longhorn audio team, led by the legendary Brian Eno (who also composed the Windows 95 startup) and later contributors, aimed for something different.
When searching for these files, you will often find them packaged in ZIP archives on tech forums or archival sites (like the Internet Archive or specialized beta OS wikis).