OK, I've done some experiments today:
1. connect a jumper wire between pin 7 and pin 4 (GND) - the audio was almost completely muted, but not 100%.
2. connect the jumper between pin 7 and pin 6 (+Vcc) - now the audio was muted completely!
So I'd guess this feature allows for a very elegant way to mute the audio during transmit - in fact much nicer than the way it's done in the current design (interrupt the input signal with relay K2).
Now I still need to find a way to somehow use the 12V at "TX" (from relay K1) for applying +Vcc to pin 7. Would it be safe to connect "TX" via 100 ohms to pin 7? In that case pin 7 would be at +Vcc during transmit, however at the same time pin 6 would then not be connected to the supply anymore (because it is connected to "RX"). I'm not sure if that would hurt the LM386.
Any ideas or other suggestions?
73, Allard
Allard PE1NWL