I think 2011 xperias use some different wiring or sth like that, I know skullcandy headset is incompatible with my arc s, and so is apples'.
Try various headsets on both phones, and see which one is more compatible, but it may be that sony decided to follow standards with 2012 line and used proper wiring.
Its a hardware 'issue', we'll have to live with it
Sent from my LT18i using xda app-developers app