I have a similar issue with the onboard audio of the M2N.
In the web admin, go to wizard --> Devices --> Media Directors. On the media director in question, select "Manual configuration" in the dropdown for audio output, and activate AC-3 pass-through. You will need to force reload the router at a minimum to apply this change, sometimes a reboot is needed.
With this configuration, I get AC-3 over fiber for all videos and DVDs, and I have stereo over fiber for music - but no sound at all in MythTV (live or recorded). In some cases I am able to make the change without a reboot - in these cases I do have sound in Myth.
I have not found where the configuration is lacking, nor have I determined what the audio output "Manual configuration" setting actually does.