Missing center channel with 5.1 sound [Linux, alsa plugin]

[IceHand]

I'm using VLC 0.8.1 on Ubuntu Linux 5.04 (kernel 2.6.10) with the alsa output plugin. Now when watching DVDs in 5.1 sound, I'm missing the center channel. I already checked the configuration of alsamixer and that should be alright, because in xine surround sound works just fine.
When I switch from stereo to 5.1 this appears in the console:

[00000272] alsa audio output error: unable to set stream sample size and word order (Invalid argument)
No accelerated IMDCT transform found

Any idea what could be wrong? And what does the "No accelerated IMDCT transform found" message mean?

Btw, I would be pleased to have an option like "Use 5.1 device if available", because I always have to switch from stereo to 5.1.

[Guest]

This exact same thing happens to me

alsa audio output error: unable to set stream sample size and word order (Invalid argument)