Mac mini 2011 HDMI audio issues

What are the known issues with HDMI audio with Lion and the new Mac minis?

I'm in the process of swapping out an older Mac mini that was connected to my home theater (Pioneer SC-05 reciever, Pioneer TV), with a new 2011 i5 Mac mini. The old Mac required use of a minidisplay port to HDMI adapter, and a toslink adapter to get audio to receiver through an optical cable. Previous setup was working fine, but with the new 2011 Mac mini swapped in with the direct HDMI connection to the Pioneer SC-05 receiver, I'm not getting 5.1 audio. The video is fine and is being passed through to the TV, but the receiver is only doing dolby prologic audio, even though the movies are Dolby Digital 5.1.

I'm using Plex .934 on the Mac. Audio on the Mac is set to HDMI. Plex is set to Dolby (AC3) digital capable receiver, etc.