Stephen and lintweaker,
Thanks a lot for your replies!
I'm sorry for not posting sooner, but I finally have sound on my system
What I had to do was:
1. Enable bypass-mode on my Harman Kardon receiver's HDMI input, so the Mac Mini / GT 230 get's it's EDID info from my Lumagen Radiance XD audio/video processor instead.
2. Edit "/etc/modprobe.d/alsa-base.conf" to include this line: enable_msi=0 probe_mask=0xffff,0xfff2 (see info here: http://wiki.xbmc.org/index.php?title...T220,_or_GT240
3. Use asound.conf, HDA-intel.conf (and general guidelines) from this post: http://forum.xbmc.org/showthread.php?t=74778
4. Unmute everything in alsamixer and use hw device 0.8 (as lintweaker correctly posted above) in Mplayer and XBMC (in XBMC it's "plughw:0,8" as custom "passthrough device")
I'm really happy it's working - and I hope I can replicate the above, if / when I have to do a Ubuntu re-install.
Thanks again for helping me out guys