modbranch_2021 is a “beta” branch on Steam, you can switch to it using “Properties” (in right-click menu of the library)
The audio issue happens because the game’s audio files are numbered and NTT copies these as-is, but the order can change between game/GameMaker versions, so installing NTT on top of a game version that’s too old or too new can cause mind-boggling effects with different sounds playing instead of the ones you would expect.