First of all you will need the files music files (.mp3) that you wish to convert into the games music file (.ogg) or (.OGM). Once you have your music files, head over to Media.IO<http://media.io/> and bulk upload the music files.
Wait until the conversion process finishes. Once your (.mp3) audio files finish converting into (.ogg) or it may change into (.ogm), select the option that says download as ZIP (.zip file)
Open the area where the ZIP Folder is located, and extract it into your S4 Leagues BGM folder, Mine is located @ C:\Program Files\alaplaya\S4League\BGM so just look for wherever your S4 League Folder is located and extract it into the BGM folder.
After you successfully extract the contents into the BGM folder, close that window and launch S4 League.
Go to settings, sound, and click a any map name and search for your song. Click the down arrow and you should be able to add that as a map soundtrack.
When you're all done, click apply and now you have your own custom S4 League soundtrack to play along with.






