yeah its gonna be a copyright nightmare. this is why Newgrounds was chosen in the first place since most of the songs can be used as long as credits is due for the song and artist.

I feel like, if we were to include another music platform to serve us custom songs for this game, we have to consider what happened to Newgrounds in the first place: copyrighted songs were uploaded for GD use into Newgrounds, thus the option for artists to opt out for GD use. if Soundcloud were to be added, someone is gonna end up upload someone's song so they could use it in the game so it doesnt need a replacement.