Yeah, basically what I messed up when I first wrote it is that I had the state as 195, so you would have to be in state 195 for the sound to stop. If you're hit out of or exit the taunt state (195), MUGEN won't recognize it because you're not in the state, so the sound would still be playing.
If it's in -2 it is not part of that state (195) and will be checked every tick of gametime separately, so if you're in state 195 and exit it early the sound will stop.
[State -2 stop taunt sound]
type = stopsnd
trigger1 = stateno != 195
channel = [whatever channel your sound was on]
I actually think it's a little annoying because you if have any other sounds on that channel they won't play at all because of the code. If only we could have MUGEN cache sound IDs so you could stop individual sounds. Just make sure it doesn't share a channel with other sounds.