I think it is hardware issue or at least so lowlevel driver-related software bug that can be considered as hardware one.

I have 2 resons why I think it is hardware:
1. Sometimes when I move and rotate headset connector in the plug it suddenly got recognized by device. This is much like that audio plug has poor electric contact.
2. My friend mentioned above reports his N9 works perfectly with Nokia’s headset, but doesn’t work with his KOSS headphones.

I think some electric contact in audio plug that must report connecting/disconnecting doesn’t work while other contacts that actually transmit sound still works. My application just simulates connection/disconnection events.

I have no idea why it doesn’t work... May be contact is so poor that sound doesn’t work too. May be some other reason. This app is pretty hacky and my knowledge of the root couse is very poor...