Mine is snd_howl. Very cutes.
(please list the name of the sound effect even if you add the .wav to your reply, because I don't trust people to not fire a ear explosion sound clip in this thread.)
knowing this wiki, you're going to get one anyways 
Thus the "also list name" since i have undertale mod tool up and can just go listen to/find the effect. 
This one, very cutes
I honestly forgot the name and I don't really wanna search for it so here you go I guess
Not even snd_whack_absurdlyloud?
JUST snd_ominous_hell_super??
Too far to search, I've got the unlisted tracks page right here within reach
As for the actual sounds, I should probably navigate to the sounds resource...
I literally just have undertalemodtool up and am searching the indexes for this.
sounds resource is awesome for big name stuff like Undertale/Deltarune but I don't trust them to have cool things. I need to learn how to rip sega genesis sounds so I can have the horrible screams bosses make in Light Crusader for bad deltarune fangame purposes.
Just for you...
snd_whack_absurdlyloud below this point. (the name doesn't lie. it's loud. play at your own risk):
It wasn't loud?
Are your headphones bass-light? That may be it. At 20% volume the thwack is super loud on my end.
Oh. That may be why.
I use earphones most of the time, so even worse for me.
Where does this even get used lol
https://www.youtube.com/watch?v=DbfazBRwhkE when Yellow lands here at the start.
here, though there's other instances in the raw code, I can't decipher them super well lol.
Ah yes, I remember now
Maybe it's also used in the Flowery bossfight when the other flowers land between phases?