Sorry for always replying so late - it's not so easy for me to free up some time for the hobby.
I have changed the resistors now (10k in the TX path) and also installed some 3,5mm output jackets to be able to also use an external sound card:
TX seems to work now with my cheap 3 € soundcard , I can send out signals and I am getting heard on WSPR. Problem is that with this sound card I can't receive WSPR signals - nothing gets decoded,
though the signal seems okay in level and on the waterfall.
With an external 10 € sound card I am able to decode WSPR signals, but I can't send. The audio level is just not high enough. I changed the vox resistor to 1 k which enabled RX-TX switching,
and changed the TX-resistor to 1k - but I still get only 5W instead of 10. I guess I should not go lower. Its loug enough when i use headphones.. I still need to investigate a little.