Ultrakill mod allowing users to use custom songs in regular (non boss or special) levels
Download and install UKUIHelper for the mod to work. The releases contain a custom song named "Stolen Heaven" for 4-1 and 4-2 by triageGremlin, but if you build the mod yourself, it won't come with any song.
To add custom songs, open "CustomMusic" in the mod folder, create a folder with your song pack name then another one inside named with the number of the level you want to replace the music from (ie "4-1","1-2", nothing more, nothing less) and add your 2 songs, one must be named "clean" and the other "battle". They must be either an ogg, wav or mp3 file or they won't work.