Not an expert on anything but as to your issue.
You mentioned that the PA worked so I would assume that the 1/2 of your mode switch that switches ground for your PA jack, External Speaker jack and Internal Speaker is working:
View attachment 61436
If that is true then check C197 and C196 (mismarked as R196) have not shorted to Chassis ground. If those are ok, then check the relay on the talkback PCB to make sure it is working OK.
Turn off Talkback - do you now have audio? If so, there is a component on the TB board that is bad. If not, then the relay is bad.
View attachment 61437
With the radio off, there should be continuity between 5 & 7 on J14 (and 7 & 2 on P14).
Good Luck