Took some research to relearn how to change the login/character screen music 'cause I'm on a WoW hiatus, but here's the no-MPQ-tampering method. Navigate to your World of Warcraft/Data/ directory and create this heirarchy of folders within: Sound\Music\GlueScreenMusic. Once that's done, drop in your desired login music (mp3 only I suppose, thought I haven't experimented with other formats) and rename the file to wow_main_theme.mp3 if you have vanilla WoW or BC_main_theme.mp3 for Burning Crusade installs.
Just did some looking around
\Data\Sound\Music\GlueScreenMusic
They changed the name a little bit
WotLK_main_title.mp3 <--- CREEPWALKER'S add
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