A bit late to the party, but maybe someone will come here hoping for an answer later.
The game seems to use two different types of files for its music. The "actual" background music that you can turn off with "Enable CD music" option from the menu is placed in Music directory. Tracks 01, 02 & 03 are "normal" songs, vocals and additional bits included. Tracks 04, 05 & 06 are "cleaned up" versions of the same three tracks and the ones mostly used in the game afaik, stripped from vocals and extra bits to their basic motive (if it sounds confusing, just play tracks 02 & 05 back to back in any modern audio player).
The diegetic music, one that actually occurs within the world (think of the short loop near the bar and the first hireable goon in starting location) is in the main->sound->world directory, five files with the cypress in their names and a number 2 through 5 (first doesn't have one, named simply cypress). These are short non-vocal pieces from the same tracks, 10-20 seconds in length. They are the ones usually talked about when people mention "overlapping": they are just as loud as CD tracks, and the result can be jarring when they're played simultaneously.
The next logical step would be to edit mentioned files to bring down their volume a notch. I̶'̶v̶e̶ ̶y̶e̶t̶ ̶t̶o̶ ̶t̶r̶y̶ ̶t̶h̶i̶s̶,̶ ̶m̶i̶g̶h̶t̶ ̶p̶o̶s̶t̶ ̶t̶h̶e̶ ̶r̶e̶s̶u̶l̶t̶s̶ ̶s̶o̶m̶e̶t̶i̶m̶e̶ ̶l̶a̶t̶e̶r̶. Setting any of mentioned tracks' gain in Audacity at, say, -6db seems to do the job. Providing the actual edited files doesn't seem like a good idea, though, at least with the album versions of the songs. They'd probably be taken down due to copyright concerns. So even in case this works, one will have to do the legwork.
Post edited September 06, 2018 by FattyJoker