Here is something I found on doom9.net about ogg vorbis on its faq. Maybe in the future hey?
Is it possible to transcode to 5.1 channel vorbis?
Yes, Vorbis supports multichannel audio. Correct channel coupling has yet to be implemented as of Vorbis 1.1.1 so in general it is not recommended when transcoding from AC3; the bitrate required to get a good quality 5.1 Vorbis stream will likely be close to as high as the original AC3 stream.