It’s an e250 2GB model. The “jack plug” is the common TRS connector ( t ip, r ing, s leeve).
When the TRS is slightly out the sound is O.K., though only right-hand channel [Wikipedia: TRS connector]:
But when the TRS is completely in the sound is (almost) inaudible!
The e250 itself is O.K. (I’ve tried other headphones), but the weird thing is that the trouble-making-jack seems O.K. as well in other devices (even in iPhone), though it is hard to tell whether is it “stereo” or only “mono”.
Any help much appreciated!