Well, those games had long development cycles, so some of the audio tracks were in fact composed way before the release date.
For example, in 1998 the first trailer for Oni features a track "Trailer" that made it into the final version of the game and
OST.
http://www.youtube.com/watch?v=IxJ8vp3vTW0
In 1999 Halo was unveiled at Macworld convention with a trailer that features "Halo" track from the
OST.
http://www.youtube.com/watch?v=6eZ2yvWl9nQ
As for Schizm: Mysterious Journey, there isn't much development history about it on the web, but I can say that it's a Myst-type adventure game with highly detailed pre-rendered graphics, and those were usually taking several years to create. So being released in 2001 it is safe to assume that the project kicked off sometime in 1998, right after the company released their previous game with the same engine. And by '99 there could be tracks/videos/screens to promote it to the magazines.
So taking all that in consideration, the tracks included on this CD could be the tracks already completed at the time and used for games' early advertisement.
So it's quite possible for it to be released in Dec 1999.