So I completed Undertale a few days ago and whenever I load the game, the ending music plays (reunited) but when I see people go back and play it, the normal music for each area plays. Am I being stupid? How do you fix it? I want to know because I want to find secrets, mess around etc but I want the normal music back. Thanks!
The OP has linked the following video in the comments to describe what they are talking about:
Observe that at 2:32, and again at 6:25, Sans can be seen. But at the end of the game, Sans hangs out with the others right next to the exit, and can't be found anywhere else. Therefore, the YouTuber did a true reset.
In general, it is not possible to prevent "Reunited" from playing throughout the game's areas after defeating the True Pacifist final boss, unless you reset the game. Normally, the option to do this is labeled "true reset" (and Flowey will give you an earful about not using it the first time you reopen the game). A true reset erases almost everything, with a small number of exceptions, including the following:
- True reset will not undo the effects of the Genocide route, because there is no in-game way to undo that.
- The secret door (see video above) is never resealed once opened.
In particular, a true reset will erase Flowey's memory, allow you to choose a new name (or re-enter the same name, if you prefer), and prevent any character from experiencing déjà vu as they do in a regular reset. For most practical purposes, it's equivalent to starting the game for the first time.
If you reset after defeating the True Pacifist boss, but before completing the game by exiting the Underground, then it will be a "regular" reset instead of a "true reset." This option is lost once you complete the game, as "true reset" replaces "reset." Like the true reset, a regular reset will prevent "Reunited" from playing everywhere.
(Curiously, Flowey is a bit confused in the "regular reset after final boss" scenario, and advises Frisk to "make friends with everyone this time." He seems entirely unaware of the final boss battle, which suggests this possibility is a bug rather than an intentional design decision.)