With CSL Music Mod you can add custom music to Cities: Skylines.
Steam Workshop Link: http://steamcommunity.com/sharedfiles/filedetails/?id=422934383
CSL Music Mod is a mod for Cities: Skylines that allows you to add your own music and radio stations. This mod has been completely rewritten from scratch for Cities: Skylines 1.6 and now fully integrates into the radio station system.
Features
You can find information how to add your custom music and how to create music pack mods with your own radio stations for other users in the wiki.