I haven't responded, because I don't have any good advice. I use Ubuntu all day long for work, but when it comes to media playback I use Windows or a dedicated player (like the PS3); most of the time the player. Even if Windows is better at audio over HDMI it even leaves a lot to be desired (like changing channel count when playing stereo audio vs. surround--it always uses all the channels you have configured).
If I were using Ubuntu for any sort of media playback, I'd limit it to being a DLNA server placed in a back room for a small client (WD TV, PS3, etc.) connected over the network. (As it is now, I just sneakernet everything out to the living room and copy it locally to the 500 GB HD I've installed in my PS3.)
