While I'm still on my break from digital art, I might as well show off something I remembered I did over a year ago, months before I started my art in the first place; my first ever music mod for a game! :)

https://gamebanana.com/sounds/46806

Basically, it's a huge chunk of SADX's soundtrack replaced with appropriate tracks from Undertale and Deltarune Chapter 1. I went through the whole of both soundtracks, picking the best and most fitting replacements for each of the original songs, both for the locations and the story.
Granted, it's not every song in SADX, but it is nearly all of the menu, stage, boss, cutscene and power-up music; basically, everything in the main stories and gameplay except the character themes!
(Oh, and the main Chao Garden theme is also changed. The other Chao stuff is untouched though.)

So, give it a try! Best experienced in a full playthrough! ;)
Also don't worry about dealing with Big, just use BetterSADX's "All Characters Unlocked in Adventure Mode" code to skip his story; only after you check his fishing tune though! ;)
(Oh yeah, you'll need BetterSADX too. It's kind of necessary for mods.) :P

Anyway, hope you enjoy it! ;)