Then I would now focus on the Mic amp - because you've ruled out PA, Mic starts, but fails - else the AM Regulator - but in either case, if SSB audio quits, or AM audio quits, then the Mic amp - it's the only thing that is left.
PA is a separate AMP in itself - and only to the Audio Amp chip - which when you look at the way things route, the Mic amp and PA's own Mic amp - share the Audio Pin 2 line going into - then splits off. And their outputs are sent to two different locations.
So, you'll need to backprobe, just clip a test lead onto that 100 ohm resistor by C109 - make sure the voltage doesn't' fall to zero as you "Audio" into the mic, for if it does, then you found the section that's causing the problem.
AND if the PA amp is still on, this can cause that same symptom, you'll hear it thru the EXT speaker, not the PA speaker (Bad talkback mod)
Near the front of your main board - the PA amp sits, but there is a resistor R157, that goes to a trace that goes and is dependent on the Front Panel switch that controls CB/PA - if R157 is still getting power, meaning that the PA switch is still "on" - doesn't mean the speaker has to be - just power - it will supply power to the PA section.
BUT - and pay attention to this - if the PA switch isn't right - Power is applied to both sections Mic and PA - and PA will "steal" the audio from the Mic amp. Takes more power to use it than the Mic amp behind it. PA takes all the power, Mic amp quits working.
Another condition (I've seen this in smashed panels) is where PA amp is wired wrong, and it receives power during RX, but when you key up - the LINE that C109 and that 100 ohm resistor feed - HAS A RESIDUAL CHARGE FROM C115 - Charged in RX mode by R157 but DISCHARGES thru R194 into the power line that has C109 in it - allows the Mic amp to work "part time" until you unkey the mike - so re-check your work in the front panel - you might have a similar scenario unfolding.
The graphic I'm giving you is part of another I've been working on to help you - but this shows the areas I'm discussing in this post. So I hope it helps...attached
