You have to make your own PK3 file with the folder path
And then place your custom MP3 in there, with the exact filename of the audio file you want to replace, so the game knows to play it.
You can look up the pathnames and filenames of the specific files by opening the assets0.pk3 file in your Gamedata\base folder, and sort the files by pathname. Look for any files with the pathname music/...
You can extract these to a folder if you'd like, but it's about 100MB of files. Your custom file has to replace the original in a custom PK3 and placed in your base folder.
I should point out though, that replacing the SP music is a little more difficult than replacing the MP music - since SP uses the DMS.dat file in the ext_data\ folder to assign locate points to the music dynamically. Your custom song might not align up with these time points, and could cause problems. If you want to get around this, disable the dynamic music feature with the command...
seta s_allowDynamicMusic "0"
in an autoexec.cfg file, and place it in your Gamedata\base folder.
Hope this helps. Another simpler option is to just turn down the music volume, and run a CD in the background.