Upon start-up of a new file of Undertale, or upon doing a Reset or True Reset, the "fun" value in your Undertale save folder will be a random number from 1-100. This is your natural "fun" value. Certain values will trigger unique "fun" events, such as the occurrence of a unique phone call, the appearance of a unique NPC, or a unique room change.
https://www.reddit.com/r/Undertale/comments/42i6db/a_guide_to_editing_fun_values/
"fun/Fun" Events
0:10 - Wrong Number Song Call
0:43 - Sans's Refrigerator Call
1:09 - Alphys's Pizza Call
1:37 - Nightmare Junior Jumble
1:47 - Gaster Follower #2
2:09 - Gaster Follower #1
2:40 - Gaster Follower #3
3:15 - Sound Test Room
4:28 - Fake Hallway Room
5:09 - Clamgirl
6:30 - Goner Kid
Bonus Rooms
7:30 - Room 123
8:41 - Room 264
9:31 - Room 326 (Dancing Dog)
10:05 - Room 326 (Sleeping Dog)
Toby Fox's Music Used During Intermissions
http://tobyfox.net/msuics/havercity.mp3
http://tobyfox.net/msuics/mystery.mp3
Toby's Music Links Broke, So I Uploaded Them to Google Drive
https://drive.google.com/drive/folders/1XLnY1jsS5LND6f9kuUsvYifHJ8k5-RLM
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Here's a couple of corrections I used to have on the video in the form of annotations before YouTube removed annotations:
- Sound Test Room can only show up after defeating Papyrus.
- You don't need to complete a neutral playthrough for Goner Kid to show up, the update that allowed capital "Fun" values to occur also removed that restriction.