Most collectors operate under abandonware ethics: if Microsoft sells Longhorn sounds tomorrow (highly unlikely), delete them.
Not all Longhorn builds sound the same. Depending on the build number (ranging from 3683 to 5112), the WAV files differ dramatically: windows longhorn sounds download wav
Note: Authentic files will have creation dates between 2002 and 2005 and file sizes ranging from 50KB to 1.5MB each. windows longhorn sounds download wav
Advanced: Extract from leaked build resources (requires technical caution): windows longhorn sounds download wav