So, all headphones play through only one side when connected to your phone. Yeah, try as abil suggested and see if you get stereo sound.

One thing that might help get an idea of the source of the problem is to move the headphone plug while sound is playing. Rotate it, remove and re-insert it. See if, at any point, sound comes through both sides for even a second.
You may have a bad connection at the jack; perhaps a loose wire inside the N900.