Good. I don't have it all down pat just yet (and I am at work right now so can't dig in deep!) but..... the way that TR18 circuit works...... basically.....
If you look to the right of TR18 that is all the bias circuit for it. It should be set up to bias it to where TR18 is off.... or only just BARELY on...
(by on, I mean the top lead of TR18 ... the collector... is internally dragged closer to the emitter....which is ground. So the way TR18 works is... when it is OFF, the collector is essentially floating... so the audio on that mic line above it is left unmolested... and allowed to pass through to TR17, the mic amp!)
Back to TR18... if you look just a little more to the right... you will see a diode (D11) and a cap (C72). Those actually form a little demodulator that takes in a sample of incoming RF and converts it to a low pass AUDIO representation. The BIGGER that signal is... the more it turns TR18 ON..... pulling the collector DOWN towards ground.... to knock the modulating audio DOWN. Think of it as an "audio regulator". When the demod audio gets bigger, TR18 comes MORE ON and tries to pull it down. When it gets littler....TR18 turns MORE OFF.... trying to let the mic audio rise up. TR18 is not biased to act like a switch... (meaning it is either ON or OFF) .... rather it is like a variable control....(meaning it turns ON little by little and turns OFF little by little). All of that bias goop to the right of TR18 should be working to make TR18 change to hold the mic audio down....thus preventing overmodulation. Right above it....VR4 (500 ohm linear pot) should be the modulation control setting.
If anything in that chain is wrong....it may be causing TR18 to turn on and stay on... all of the time..... which would keep your audio down..... ALL OF THE TIME!
As mentioned that cap C69 that ties on to the collector....could also be shorted. That would have the same effect...because it's other side is ground. So...if it is shorted.... it will pull that mic audio down too.
Good luck.... see what you can find out!
Bob