If it's really hard-coded into the client to play eqtheme.mp3 during load screens and character creation and selection, then the only way to change it would be to make changes to eqgame.exe, or one of the .dlls if that's where the coding is, and that's beyond the scope of my customization work.
On the other hand, while I've had a bunch of non-EQ real life things taking up my time lately, I've managed to finish up a working Eff2Emt converter with a graphical interface.
Give it a test and let me know how it works for you:
http://www.shendare.com/eq/emu/eff2emt/