This mod changes the music used for the game's main and in-game menus.
Extract the file "zzz_phan_gui_menu.pk4" from the .zip archive and place it in your Doom 3 base folder. To uninstall, just remove it from that folder again.
The music tracks are by Sunlight Defect specifically "Voidreaver" and "Singularity". I just edited them some in Audacity (~30% slower, reverb mainly). Their music has a really great hard-driving feel!