I'd found this problem consistently with Amarok. While playing tunes in my library, Amarok would simply stop on the next tune and gave a pop-up saying "ID3 tag demuxer". I searched everywhere for a solution. What I did was to simply remove gstreamer good 0.1 and then re-install it. I don't know what this did, but magically, all of my tunes play now without problem.
While recognizing that my solution may not help all, this did work for me and was very much simpler than most more complex solutions on the net that didn't work for me.
I hope this helps others.