Maybe you could use a mono cable? I remember myself using a single earphone plugged into a walkman (yes - a blast from the past) and you should get a physical downmix. Another thing that I have experienced was connecting a samsung stereo headset to a nokia phone which had a different depth of the audio jack and the sound was terrible unless I pulled the plug a bit out so it sent the sound on both channels. A third thing, when I plug something onto the audio jack on my laptop I get a pop up asking me what I have connected (headphones, headset, speaker out,..) so it sure will change the way how the sound will be sent to the jack.
Hopefully I did not misunderstood your problem :)