I assume the mics and headsets would still connect directly to the PA system, with other A/V sources going through the 950 and then from the 950 to the projector and an audio input on the PA.

If that assumption is accurate, you could probably most easily use the Tape In since it would just require a standard pair of RCA-terminated audio cables. The Line In could also work, although you'd need a 1/4"-to-stereo RCA cable. In either case, I'd connect the 950's left/right pre-amp outputs to the PA system and set the 950 up so that it only has the front speakers enabled (no sub, no center, no surrounds). You'll probably need to tinker around a bit to find the right volume setting on the 950. Otherwise, you can then connect any source you want to the 950 (DVD player, CD player, VCR, tape deck, laptop sound card, cable/satellite receiver, etc.), use the video switching as you would under any other circumstances to feed video to the projector, and have the audio go to the Tape In or Line In of the PA system.