Figured it out. For anyone who reads this, use File Garden instead of Dropbox since it's easier. Just grab the .ogg file of whatever song you want (other formats may work but I used .ogg), go to your garden in File Garden, click upload, then copy the link to your clipboard and paste it into the game. Unfortunately, many songs (like the one I was using) have fadeouts at the end and you can't skip the last part like the first so you can't really get a good loop for those songs.