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Although the problem is not fixed yet, we can conclude that it's not a
software issue.

Funny, I had exactly the opposite problem, initially.
Sidetone was cut down during first second, after one second it was OK.
Probably because the LM386 remains on for some time until the 470uF
electrolytic is discharged.

I could improve it by adding the 47 ohms series resistor, to create some
seperation between the two outputs. It's not a very elegant solution but
it worked on my radio.

But it seems you have exactly the opposite situation: first second is
still OK, but after that the sidetone is cut down.
What's your speaker impedance and power? I'm using a 8 ohms / 5 Watt speaker.
Do you have the same problem when you switch to headphones?

Any ideas/suggestions/comments from other builders?

73, Allard PE1NWL