I have noticed a behavior similar to what the OP is describing:
Step 1. Recording starts with AC-3 2ch audio, so MythTV renders a PCM
2ch stream to send through audio output
NOTE: I have observed this behavior on my own setup by looking at the
indicators on my a/v receiver. I have upmixing disabled and Dolby
Digital Passthrough checked.
Step 2. Somewhere in the recording (possibly in a commercial), the
audio stream changes to 5.1ch. MythTV switches output to Dolby Digital
Step 3. When the main program returns from commercial, audio output
remains Dolby Digital, though 2-ch this time.
I have noticed that the rendered PCM stream does seem "louder" than
the same 2-ch audio passed through as AC-3.
To the original poster, does this explanation appear to work with your
mythtv-users mailing list