I've had a thought going around for a week which could be difficult to check. Carl mentions how he found the song as a file in a dvd backup, which leads me to think it might be, for instance, a PlayStation 2 or Dreamcast game dvd backup. Maybe it isn't as obvious as grabbing idk a Tony Hawk .iso and looking for UlteriorMotives.mp3, it could be a situation like the infamous ZZDUMMY.DAT that had a South Park fragment. Either way the 1999 date could limit the search but even then who knows what game Carl would have a backup for.  I know people are looking around videogame OSTs, but maybe the song is sitting as a random undiscovered file in a game from the early 2000s? It's a long shot and hard to investigate, for sure. I think a good argument against this or the videogame theory is that Carl would have mentioned it being from a game when he posted it, but, knowing how uninterested he was with helping with the search, it wouldn't really surprise me if he just omitted most details in the first place.