This mod is now archived and deprecated in favor of USTManager which should release anywhere between January 1st and 3rd (the repo will be private until then so the link won't get you anywhere)
Update: As of v0.8.0 (currently unreleased), no longer requires UKUIHelper to work
Ultrakill mod allowing users to use custom songs in regular (non boss or special) levels
To install, just unzip the release you downloaded in your BepInEx plugins folder (or "UKMM Mods" if you use UKMM). 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.
To build UKMusicReplacement, you can either use Visual Studio or Visual Studio Code and dotnet. When building is finished, you should have a dll file. To use it, just put it in a folder and put that folder in you BepInEx plugins folder or "UKMM Mods" folder if you use UKMM.