Re: No audio from HDMI (MCP89 / Geforce 320M): Reason: no connected devices detected
Can you please post the complete output from mplayer (also, add the "-v" command-line option). This should tell us which audio frequency your file is, or which audio frequency that MPlayer is sending out.
Looking at the "eld" file, it looks like your receiver claims it can't support PCM audio. Can you please run nvidia-setting, select the page for your display which will be named something like "DFP-1 - (HCG Harman AVR)", click the "Acquire EDID" button, and post the resultant file here.