On my Asus M3A78-EM running Ubuntu 9.04 I use  XBMC and Boxee for media playback. I recently had the need to output audio over s/pdif optical and via HDMI due to the fact that I have a HDMI switch to send the output to two separate LCDs.  I also have a MSI N9400GT-MD512H GeForce 9400 GT installed in this box to perform VDPAU video hardware acceleration.  As you may or may not know this GeForce card requires a 2 pin s/pdif cable to be run from the motherboard to video card.  In order for HDMI audio to function you must go into your bios and set the `SPDIF OUT mode` setting to ‘SPDIF Output’.  One the system is back online, be sure to disable Pulse Audio as it does not play well with Boxee and XBMC. Next you need to have the latest ALSA v1.0.22 installed, a nice upgrade script for Ubuntu can be found here . To check what version you have installed check the following file:

htpc@Electric:/media/sda3$ cat /proc/asound/version
Advanced Linux Sound Architecture Driver Version 1.0.22.1.
Compiled on Feb 21 2010 for kernel 2.6.28-18-generic (SMP).

Ok, once you have the latest version installed you need to configure your /etc/asound.conf  file as follows:

pcm.!default {
type plug
slave {
pcm “spdif”
rate 48000
}
}

EDIT 2/22/2010:  I found that under the latest Boxee Beta 0.9.20.10710 I could not get sound to function at all!  On a hunch I deleted the asound.conf file above and everything including navigational sounds worked like a charm . XBMC also function perfectly, so I really question the need for the global asound.conf file above.

You may also want to check in your home folder to be sure that you don’t have any custom asound configuration files, you really should remove these as to not cause any confusion.

Go ahead and do a reboot if you have not done so. Once the system is back up you should run alsamixer and turn all your volume levels up, pay special attention to the SPDIF levels  be sure they are unmuted. An unmuted channel will have a ‘OO’ and a muted channel will have a ‘MM’.  Once you have your levels set run ‘sudo alsactl store 0’ to save your settings.  Lastly, in Boxee and XBMC set your audio settings as follows:

Audio output———————————-Digital
-Dolby Digital (AC3) Capable receiver—-off
-DTS Capable Receiver———————–off
Audio ouput device————————–default
Passthrough output device——————iec958
Downmix multichannel audio to stereo—on

These are the setting I have to use for everything to work with an older Sony XBR1 I have, the TV does not seem to be able to support Dolby Digital or any special formats.  Good Luck.