I don't know how relevant this still is, but what many of your posts seem to point out yet not explain (and sometimes contradict), is that so long as you're not publishing or selling it, so long as you're not reaping any (financial) benefits from reusing the original work, there is no copyright infringement going on. So is it "OKAY" to use Quake's sounds in your freely downloadable mod? Sure.
Perfect example: www.goldeneyesource.com A nearly exact remake of the N64 game from Rare "GoldenEye 007." The lead developers at the mod contacted Rare and were told it was completely legal to do what they're doing. (And they are using some of the original sounds from the N64 game.)
I very well may be wrong about the way I understand "infringement," but no one's been able to explain it any better with any real corrections.
PS, really like the idea of GR:CS hope it makes it around for the best. =)