When I listen to audio through headphones on my Galaxy S2, there's a phase-cancellation effect that attenuates any centered signal (so I often get most of the music but little of the vocal, or on podcasts with voices panned center I get very little anything).
But if I listen in the car - through a 3.5mm male-to-male aux cable - or on the internal speaker, the audio sounds fine.
I've tried four sets of headphones, with both two and three contact jacks.
Also tried hard-resetting the phone, and this didn't have any effect.
All these headphones used to work fine. I started having this issue earlier this week, around 2/28.
