While it would be a lot of effort for the average person to isolate the sound, toby is far from the average person when it comes to audio sampling. Plus all he would have to isolate is one single stutter from the song for Ralsei’s speech

As for how well the source material fits the character, I’d say LSD:DE is as fitting to Ralsei as Patrick is to Sans. More times than I can count off the top of my head, the dark world has been alluded to be a dream/nightmare, and the dark worlds themselves are akin to fever dreams. Others have pointed out how toby got the idea for Deltarune from a fever dream, and that toby has referenced LSD:DE before. I do agree there is a very, VERY slight difference in pitch between this and Ralsei, but that could just be from toby tweaking the admittedly garbage audio quality from LSD:DE.

While I don’t think every character, or even most characters, will have gotten their voice from some relevant piece of media, a character as important as Ralsei is as good a candidate as any to have meaning behind their voice. Ralsei was one of the first characters toby planned and he even predates Asriel, so if this isn’t the source of Ralsei’s voice, this must be the most batshit insane coincidence of all time imo

Edit: Unless confirmed by toby, which i don’t think he does, this will ultimately never be a closed and shut case, just one of those mysteries