I tried using a standard stereo patch cord from the MRX audio port (with mini adapter) to the audio input of my PM501 intercom, and I couldn't hear the warnings thru the headset. I tested the audio port with an MP3 player with a stereo cable and the port works fine.

I haven't tried the cable provided (headset cable), and will do that and report back.

But I'd really like to just plug it directly into the intercom with a simple patch cord, so I want to sort out why that's not working.

Is the audio output on the MRX wired out to the tip or ring (or both) on the jack? My intercom audio input is mono.

I can see through the plastic case that the jack is wired out (red wire) to a pin on what looks like an unpopulated test connector on the circuit card. Is it possible that the wrong pin location was wired out? What is the correct location - I can verify visually to rule that out. A white wire appears to pick up a return off another component pad.

Is it possible that the input impedance of the PM501 audio port is not what the MRX audio output wants to see? What impedance does it want to drive into?

I'd like to get this sorted out without having to return the unit, as I have a trip planned in a few weeks. Seeing as it's to Dallas, if I can't get it resolved for the trip down, I'll pop into your shop and see if I can get it looked then for the trip back.