The sound effects in Ultrakill have always felt super satisfying to me, and even here in this speedrun where it gets incredibly chaotic you can still tell each sound apart and every one of them has purpose/direction. The use of tape-stop/starts when the game slows down, doppler when you zoom past an enemy, and the hitstop (freeze frames to add impact), are all implemented so well to me that it makes both slow and super fast gameplay feel amazing to me.