"Dating Start! (FM Version) (In-Game Mix) - Undertale" is a high quality rip of the in-game mix of "Dating Start! (FM Version)" from Undertale.
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There isn't a in-game mix of "Dating Start! (FM Version)" from Undertale; "Dating Start! (FM Version)" is exclusive to the Undertale Collector's Edition Soundtrack.
This rip plays the original version of "Dating Start!" from the advertised game, with it arranged to "Sunshine, Rainbow, White Pony", referencing the rip "Dating Start! (FM Version) - Undertale".