This will tell you how to change the music it plays when you login to World of Warcraft.
Choose a song (must be mp3) and rename it "bc_main_theme.mp3"
Now go to you're data folder and create a folder called sound.
Now inside that folder create a folder called music.
Now go inside music and make a folder named GlueScreenMusic
Sorry if its a repost i checked the threads that it says to check b4 posting.
The credits of this go to LS12 at ac-web
Filebeam - Free Fast File Hosting is where you can download the Wotlk login music.