Vorbis - Mar 2005 - Quality Problems with 6 channel ogg files

I have tried to make a 6 channel ogg file from a DVD in the following way:

1. Extract the audio from the 5.1 DTS track on the DVD to 6 wav files
2. Use BeSweet to merge the 6 wav files into a 6 channel wav file
3. Encode the 6 channel wav file using oggenc (latest megamix2, but 
other encoders produce the same result)

Result:

The rear right channel produces severe artefacts. Yet, if I encode only 
the mono wav file representing that channel, there are no artefacts. The 
quality setting doesn't matter (but most tests were done with q6.0).

Another strange thing is that if I switch the left right and the rear 
right channel and encode, it is still the rear right channel that 
produces the artefacts.

I have uploaded a test file here: 
http://www.daimi.au.dk/~michael/other/test.zip

It contains sample wav files to reproduce the problem as well as the 
tools I have used.

Can anybody tell me what I am doing wrong, or what I need to do in order 
to produce a 6 channel ogg?

Regards,
Michael