I have heard a few complaints about that problem. Some of them will have no receiver audio at all.
The culprit usually turns out to be a transistor that completes the ground circuit to the internal speaker AND the external speaker jack, both.
If you won't be using the PA function, simply shorting across it will probably take care of the problem. The callout number is Q73, and it's located immediately to the front of the external speaker jack.
It's a surface-mount part, like nearly everything in that model. Not a project for a beginner.
Somebody should shoot a couple of pics of this one. Gotta figure there's a place to put a jumper wire that wouldn't risk damaging the other nearby parts. Haven't run into one of those first-hand, yet.
73