Neither the music nor the DSL startup comment are found in the Sounds folder.
If you open up the Music folder, you should see three folders: Main UI, Team HQ, and Workshop. All of the MP3 files found in the folder that corresponds to your current screen will get played alphabetically according to the filename and will repeat as necessary. The startup voice is actually "./Music/Main UI/ADSLTC.mp3" and since it begins with a capital A, it plays before "dsl-DiE oXide.mp3" and "Enter the Battlefield.mp3." If you wait in the Main Menu for 7 minutes and 27 seconds, you'll hear it play again.