I modified the original soundtrack, due to some music tracks not having good audio quality. It was a little too loud, and I didn't like how it sounded. So the solution was to turn the .mid tracks into .ogg
However, it is true that I could keep the soundtrack as .mid and use a soundfont, but I don't think this will change anything.
Whereas the stage select menu, it's not there, because Sonic Jump Anniversary is currently a demo. The full game version will come out later.
In any case, thank you for the comments. I will take all into consideration when the weekend is done !