Ok - after a lot of testing and research I fixed the problem for me (FP4):
In Settings (Einstellungen) > accessibility (Bedienungshilfen) > mono audio: switch on “combining chanels …” (Kanäle bei Audiowiedergabe kombinieren)
Incoming Skype-call voices sound totally normal now.
(The official Skype Echo Test contact is “echo123” - there are a lot of scammers named “Echo Sound Test” …)