Think about it. The OP alternately says the mic when pushed in for ptt is in receive indication, not transmit, but also says the radio is mute in receive. If I had to literally take this at face value off a repair ticket (which I wouldn't), my first thought is to check the center/common wire on the T/R switching in the microphone. If that wire was broken/open which usually is tied to ground on most electronic switching radios, the ptt line would never go low, so no red transmit indicator or transmit an at the same time in receive without the mic keyed, the ground return on the secondary side of the modulation/audio transformer would be open. You'd still get the green light for receive, but the audio would be dead. That's assuming this is wired like this and one of those radios that have no receive audio when the mic is inserted.
Again, I never, ever would take a customer's description as valid unless cleared by an installer as a valid description of the problem, but immediately substituting a known, working mic before opening the chassis would be my first diagnostic and reasonable suspicion with usually tampered with equipment like CB's that something may be amiss in the mic or mic connector. Not something I'd expect to find with a commerical radio as they're rarely messed with by the customer.
Just saying.