I don't think that should matter as the sound works properly with LinuxMCE, the problem is that MythTV is not setup properly for sound output.
The sound chip I'm using is built onto the mother board of a M2NPV-VM, one of the recommended boards. I purchased the daughter board ($16) that provides optical and coax PCM output (dolby digital). This is hooked to my home stereo. The sound is very good when playing a DVD or audio and non-existant when playing MythTV.
I'm using the component out port at 480P or 780P but I get digital sparkles in the background on dark images. HDMI or DVI is the way to go.