Multichannel audio with XBMC, center channel problem

I'm having an issue while using XBMC to play video with AC3 or DTS audio streams. The center channel sound seems to be redirected to the left or right channels, sometimes it's left, sometimes right!

I googled the issue and got to the XBMC support forum, where a few similar problems were discussed. Needless to say I've tried every possible combination of output to HDMI, Opt/coax, Analog etc without success.

It's fine for non-multichannel streams. Anyone have any suggestions?