Related info FWIW ... today I purchased a "Headset Jack Adapter - 3/32" to 1/8" Wireless Phone" from R.S., part no. 274-004. I had a weird experience in the store, testing the adapter. At first it did not work. Then the sales person offered to try a standard mini-to-micro adapter. That gave me one channel, per my previous experience. While she was looking for other alternatives, I tried the aforementioned part 274-004 again, and voila! it worked. No real clue as to why, at first. Later at home I plugged in my headphones and again it did NOT work. Arg!
I experimented with several configs, and finally found the solution:
Step 1: Plug in mono headset or standard micro-to-mini plug adapter
Step 2: Play media
Step 3: PAUSE media
Step 4: Plug in stereo headphones
Step 5: Un-pause media -- voila!
Stereo media through headphones.
Now, I suspect this isn't all good for the BB. I suspect it may be shorting the microphone circuit. In between songs I get a little "snap/click" kind of noise. Dunno if that's endemic to the BB's media player or if it's a symptom of electrons going where they shouldn't. But I'm using it in this config and will see what happens.