I don't normally mod games in general, but I want to mod the normally unused song Pizza Mayhem into The Crumbling Tower of Pizza as going for 101% will still not have the entirety of "Bye Bye There" playing in a speedrun attempt of CToP, while Pizza Mayhem will just barely hit its loop as it's only around 3.5 minutes long.
It'd just feel really cool if that was playing on a good run so I'm really hoping the answer is yes. The mod in question: https://gamebanana.com/sounds/66594
Unfortunately no, we wouldn't allow it. Any and all mods to the game are disallowed for verified runs. That includes anything that replaces music.
You could, however, mute the game music and listen to your own music through other means. Just make sure game SFX is clearly audible.
Noise has arrived to the leaderboards, alongside Swap Mode, both in normal game and CE. You can now submit runs as the gremlin! Go ham or go stupid! Whichever works! No additional rules are given to either categories, yet.