Just started today to link my laptop computer to my Elecraft KX-3 (S/N 2641) for use with amateur radio PSK31 software programmes.
Have successfuly linked 'KComm' and 'Ham Radio Deluxe' both with receive audio and CAT control working, however like you when I also connect directly (using standard PC audio leads) from the laptop headphone audio out socket to the KX-3 microphone input socket the KX-3 radio enters TX immediately.
I haven't figured out a way to overcome this yet, looking at the Elecraft 'cable set' they too use similar 3.5mm straight cables, so I can only assume they are wired slightly differently.
Regards
Philip G0ISW
http://www.qsl.net/g0isw