I've dabbled in engineering (built studios, patched audio into xmtrs., etc.). What the airchain would look like would be this: STUDIO->EAS->Stereo Processor->Transmitter (with a stereo exciter which may also be around). Anyone listening on a mono radio would only hear the mono signal because the way C-QuAM works is that there's a left plus right (mono) signal and a left minus right (stereo) signal riding out of phase. So, here's a checklist:
1. Are the air & production boards stereo?
2. Are the audio playback devices (computers, C.D. players, cassette players, record players, etc.) bridged to mono (left and right tied together) either in the back where the audio wires plug in (using a Y connector) or in the board? Basically, an easy way to check all of this is to play a song with extreme stereo in it (something like "Lightning Strikes" by Lou Christie) & if you hear the left & right coming from the left and right as in the song, you're stereo up to that point. If it sounds like it's coming from the middle, you've got mono somewhere.
3. Assuming that everything through here is stereo, the E.A.S. should also not be bridged. It should pass left and right independently.
4. The audio processor would have to be check to make sure it's clean. If so, it can go back to being the primary audio processor. What kind is it & what are you using now?
5. If the transmitter is still the one for stereo (assuming it was at one point), there should still be some exciter or card that generates the stereo. If it's made after 1993 it'll be C-QuAM. Before 1993 it may be one of the other systems which were competing at the time. Those were: Kahn/Hazeltine, Magnavox, Harris. Believe it or not, all of this could be check within 5 minutes!