How do I adjust the volume of custom music files to the volume level of the original duke3d music files?
Don't know if this is a feature in EDuke.
Maybe you should pull out one of the original music files and have a look at it in a sound editor?
"Goldwave" is an alternative.
In eduke the user can control music volume separately from sound so they can put it to what they're comfortable with. I.e I don't see any real need to change the volume level of the actual track.
There's no way to define the volume of the music with eduke by the author, however if you have a more modern format like Ogg then you should be able to edit the volume level of the music itself in a sound editor. Audacity is pretty good.