5. spamton_neo_after.ogg
For some reason, it looks mirrored horizontally. I don’t think any other ogg file in this folder looks like this. Some words or message could easily be hidden here, but I wasn’t able to find them. (14/18)

4. THE_HOLY.ogg

THE_HOLLY.ogg plays when you close the fountain. Maybe I’m going insane, but it really looks like left half of the Gaster’s face. If you will compare spectrogram to the song itself – the part with Gaster’s face is actually the choir part. (12/18)

Also, it contains a strange line in the middle. By trial and error, I discovered that the line appears when you combine two songs in audio editing software - like audacity. That may indicate that d.ogg was made out of two different segments of some yet-unreleased song. (10/18)

d.ogg is playing in the snowgrave route when Noelle meets berdly and mistakes berdly for an enemy.
What’s interesting is that the spectrogram contains some noise in the top right in the second half of the picture, despite song just being pitched down in the second half. (8/18)

3. w.ogg and d.ogg

W and D are two letters of the three initials of the name W.D.Gaster. There is no file named g.ogg.
w.ogg plays at the start of the first chapter, when text says “and your name is”. Though, in the game it is either slowed down or sped up partially (6/18)

2. Wind.ogg
wind_highplace.ogg suspiciously sounds like a changed version of wind.ogg, with only addition being spamton’s face.

Spectrogram of wind.ogg, shows strange arc in the bottom right. It looks like a smile (Mike) or the Delta Rune (wings and the circle below) (4/18)

1. wind_highplace.ogg
wind_highplace.ogg contains a picture of spamton. It is only visible if you render spectrogram at 100 width and very big height (around 1300 – 1500). This track plays at the roof in the snowgrave route, before entering Noelle’s room. (2/18)