It's incredible. The quality of the audio is MUCH improved. The stretch room has some new audio effects (creaking noise, sound of bats, etc...). The voice of the Ghost Host travels all around the room. The narration and all sound effects are much louder than before.
Each doombuggy has three speakers (L, R, Center) and the Ghost Narration also moves in and out of each speaker, making interesting induction recordings. There may be two sentences of additonal dialog, one as you exit the stretch room and the other as you exit the doombuggy, but I haven't confirmed this against a live recording of the previous version.
I am usually not a fan of rehabs, but this is by far the best that I have seen. In fact, I would say the new version of the HM is better than the original (imo).