Yeah, it's happened before, a game rip not existing. The few cases of it that have happened, the submitter ripped the track themselves and just hosted or attached it without having to be prompted. If they didn't already do that, we just email them as ask.
As zircon just mentioned to me in #judges, "If we don't have access to the source, we can't judge it, so, it then becomes remixer's responsibility to provide the source". djp also added, "The policy is also case-by-case for rejecting games due to extreme obscurity/"indie"-ness." So if the game is just someone's Newgrounds game with original music, but the game isn't mega-popular, then we'd pass on it.
I supposed we can add a sentence to the guidelines on that, but I honestly can't say I've seen the need in 6 years of being a panel member. The worst case scenario is just that the submitter can't find the original audio and we have to do the legwork. We've yet to have something submitted from something that absolutely can't be tracked down.