P4S533-MX & SoundMax won't output 5.1 sound through SPDIF

Running Windows XP: I've specified 5.1 speakers in SoundMax and connected PC
to the Dolby digital receiver using motherboard's coax SPDIF. In SPDIF
settings I checked both PCM and AC3 checkboxes. When I run SoundMax's
sound test, which circles the sound around you by moving from one speaker to
another, I get complete silence when the signal is outputted to center or
rear speakers. Only two main channels produce sound. (I assume test
signal is AC3). The same happens when I configure my software DVD player to
output 5.1: two main channels get decoded while remaining are silent. For
example, if a movie character talks through the central channel, there's no
talk sound - you can only hear what's on main two tracks trough main
speakers. All of this sounds like SoundMax outputting only 2 of 6 channels
through the SPDIF. I'd appreciate if anybody told me how to configure
SoundMax to output all 6 channels.

Thank you,
Vlad.