Not to self advertise or something but if you liked this video, consider checking out my music? There is some Undertale stuff as well c:

Sylver and I were experimenting with this particular Gaster file that plays when Gaster's entry shows up in Undertale. To our knowledge, nobody else has been able to find out what exactly the sound is supposed to be. However, after Sylver speculated from another video that we should speed up what sounds like "laughter" (as said in another analysis video), we discovered that the audio file was actually Muffet's spiders' laughter audio files slowed down by an immense amount and reverse and looped over again. Watch the video for yourself.

https://www.youtube.com/watch?v=8AeYyry__CY
^here's where we built some of the research off of.

(Edit): Some already found this, however, this will be the official evidence.