Hello, there are no game mods for the game, however there are music mods that you can download and apply. Some of them are on itch as well.
https://thegamerdownloader2020.itch.io/custom-soundtrack-for-sonic-3d-in-2d
https://kitty2135.itch.io/sonic-3d-in-2d-the-extra-soundtracks-with-audio-loop-f...
https://ferchoroger.itch.io/remastered-soundtrack-sonic-3d-in-2d
http://www.mediafire.com/file/q4fure0469dw7k9/Sonic_3D_in_2D_HD_Remixed_OST_Mod_...