[Frodo] Front contains rear channel

Hi

I'm rugging Frodo Beta1 with Ubuntu12. Audio chip is a HDA ALC888 with analog 5.1 output.
speaker-test -c 6 -D surround51 -t wav
is running fine, I get correct output to all of my speakers.

I have a vob-file with ac3 encoded 5.1 sound. The problem is the rear channels are mapped to front and rear channel.
Center and LFE are fine. Also signal from front channels isn't mapped to rear channels.
In xbmc output is set to analog 5.1, Stereo to all speakers is disabled.
I use pulseaudio as playback device in xbmc

Any idea why the rear channel is also outputted to the front channels?