This page contains a tutorial on how to use custom songs, here is a list of every usable custom song
Credit for every single song goes to @jamZgator, links to check out his very epic stuff below
Custom Songs within Trigonometry Sprint use an ID system, with each song# being visible on the custom song list page above
Simply take the part number before the colon, and paste it into the URL field in the level settings (shown below)
also note that custom songs will not work correctly on versions older than 1.02_02, and are still strange on the remaining versions older than 1.1