To start, I'd look at listening to the PA side of things - see if the Compression distorted stuff starts back at the Mic amp...Talkback or EXT Speaker to PA jack and LISTEN at the Mic level - just have the volume turned down to listen at a comfortable level to possibly hear for those nuances they complain about.
IF you hear them, these compressed "Sounds" or weird-ed effects then the AM Regulator is not the cause - it's back at the Mic amp and its circuit. IC7 will need some TLC.
Look for mods, replace - repair - remove as needed -
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But if not...meaning you can't seem to recreate the condition - make sure you set their mic up to do this -- verify the mic and or mic amp is not injecting its own colors into the audio.
Once you're satisfied with that, let's look at the Area Of Concern that brought you here...
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There is one cap -
C214 - that normally is used to SMOOTH off audio "bass" and also be a Hi-Cut filter
- it is one last step in the audio processing line.
D93 is not - but C214 is still used for this purpose - it also gets hammered by wild voltage swings - might want to just do a simple swap out - remove and put in a different value cap to hear the effects I mention - you'll see / hear how this particular cap affects audio - while this radio is exhibiting this condition - it can help you determine if the problem is with input drive versus the audio and bias drive instead - trimming voltages on the Main board listening to the audio - you can develop a profile from these value changes to help you troubleshoot the problem.
