When folding the Dolby Stereo mix down to mono, I neglected to decode the surround channel separately and add it back in. As it is, any surround information has been phase cancelled and is essentially absent from the mono folddown. For this particular exercise, I think the track as it is, serves the purpose well enough - the sole exception being that the discreet echo on the dialogue in the chasm is not heard.
It is simple enough to fix and reissue if anybody wants me to.