Songs that will be / already has been used for DiscTux, a mod of SuperTux that has new levels, cutscenes and more that replace the normal SuperTux levels and cutscenes and more.

Description on my YouTube videos about it, hopefully easier to understand:

DiscTux is a mod of SuperTux that answers a question nobody has or ever will have, "What if SuperTux was like Sonic CD?"

Each world is called a round and each level is called a zone because Sonic CD. There are 3 zones each round, with the third one being the boss.

There's time travel and you need to find things in different time periods to unlock the boss fight so you can defeat the boss to progress through the mod.