In 2005 Bungie told Claude Errera that he could put up all of the original Marathon content for free download and redistribution (with the exception of the CD audio tracks from the original M1 CD, which Alex Seropian has asked not be distributed). See trilogyrelease.bungie.org. Since then we've all been distributing Marathon content with impunity and wild abandon.
However, Claude never got a specific legally-worded license from Bungie, despite several requests for one over the years. So basically all we have to go on, legally speaking, is Claude's word that Bungie said it's OK, and the fact that Bungie is clearly aware of what's going on and hasn't complained about it yet (IIRC they've even linked to it on a few occasions).
To be safe, you should probably assume noncommercial/attribution, but otherwise, go wild.