OK, got it to work today plugged directly from the MRX audio port to the audio in on my PM501 intercom.

It seems the problem was (and still is) with the audio jack on the MRX. When I plug the stereo plug in, it tends to back out about 1/16" from the jack, and that opens one of the connections (likely the tip). I tried a bunch of stereo plugs from different manufacturers and they all do the same, so I have to assume it is a problem with the jack.

If I push on the plug, it works fine. If I don't, it backs out and then doesn't work.

I'll be in the Dallas area in a few weeks so will stop in and have it looked at then.