Current ATI DVD Decoder versions do not - only the DVD Decoder include with
the Home Theater Enthusiast software here http://shop.ati.com/product.asp?sku=2550132 permits that. Apparently its
possible to use that DVD Decoder with newer versions off MMC and get working
5.1 sound.
May I suggest Media Player Classic, which does DVDs, XviD, subtitles,
multiple language streams, 5.1... all without the bloat and fruity interface
of the ATi DVD player.