Finally, the moment has arrived! After trying on at least five GNU/Linux distributions and FreeBSD, 5.1 surround sound via S/PDIF (aka. IEC958) finally works, on the latest stable version of the new wunderkind of distributions, Ubuntu Gutsy. The instructions were easy enough, and what follows is a version you can pretty much copy to the command line. Note that these are for ASUS P5K-E motherboards (which have the Analog AD1988B sound chip) – they may not work for you.
sudo apt-get install linux-ubuntu-modules-2.6.22-14-386(look out for newer versions later)
sudo depmod -aAe
sudo modprobe snd-hda-intel
I fired up a DVD, a Dolby Surround demo, and Doom 3 with the latest patch and
+set s_alsa_pcm spdif +set s_numberOfSpeakers 6, and whaddyaknow! It all works! Even the manufacturer’s Windows driver doesn’t support optical surround sound.
Thank you developers!
+set s_alsa_pcm spdif +set s_numberOfSpeakers 6 also works for Enemy Territory: Quake Wars. Yay!