Smaragd,
As "a" is displayed, I'd expect the Walkman audio to get through, so the question is, why isn't PAW? Doesn't appear to be wiring, because it's OK when "v" is displayed. Is the PAW audio at a similar volume to the Walkman, I.e. Could it be lack of drive level? Presume you have adjusted the input sensitivity of the aux input to max because PAW (Rpi) audio output is pretty low.
Hmm, just noticed your earlier post when you said problem persists when Walkman level is lower than PAW. Tis a strange one! I think I'd still try setting the aux input level to max sensitivity to see what happens, if you haven't done so already.
I believe it is possible to turn off the VOX so mic is on all the time. Does this have any effect?
The Rpi audio output is 4 pole, I.e. Tip (LH audio), Ring 1 (RH audio), Ring 2 (Ground), Sleeve (Video output). If you plug in a 3 pole plug (same length as 4 pole, I believe) then all you are doing is grounding the video output, all other connections should remain the same.
Regards,
Chris