Are the audio dropouts spaced uniformly in time -- say once every 10 minutes? If so, then the odds are the Panasonic is failing the periodic, background copy protection check that happens over HDMI. Usually that check happens in the background and you never notice it, but if it fails, a retry happens and audio is muted while that's going on. The minimum time for an HDMI retry is on the order of 2 seconds. If more than 1 retry is needed to get things working then the delay goes up in multiples of 2 seconds.
The first thing you need to know is that HDCP (the copy protection protocol layered on top of HDMI) *LIKES* to fail. It is finicky by design. Often such failures are due to marginal cables, but you seem to have eliminated that.
What I think is going on in your case is that the Panasonic is not responding fast enough to the check on HDMI 1 (which is a fairly complicated sequence due to going through 2 video processors in the OPPO). Evidently it isn't failing BY MUCH because things are working in the simpler case on HDMI 2 and also on the 83. It may simply be that your Panasonic is not compatible with HDMI 1 output from the 103. There are lots of ways this can happen, only one of which might be fixable by you -- and that would be to check if there's a firmware update available for your model of Panasonic. These sorts of problems arise because the HDMI chips come from different vendors, who interpret the HDMI specs differently, compounded by the firmware that operates those chips, perhaps not being optimal for speed.
Since the Panasonic is ALMOST working with HDMI 1, it is also possible that HDMI-related settings in the Panasonic might alter its response time. For example, try turning off anything having to do with remote control over the HDMI cable in the Panasonic (HDMI CEC is the real name, but Panasonic might call it something else). On some displays, HDMI CEC is enabled automatically on only 1 input. If that's true about yours, try another input. Also turn OFF HDMI CEC in the OPPO.
Some displays also spin their gears more than makes sense when sent 1080p/24 video. Try setting 108p/24 Output OFF in the OPPO.
You say you already have the OPPO set to LPCM audio output. Also check that you are using an explicit video output resolution choice -- e.g., 1080p instead of AUTO or Source Direct.
If you've not done so already, also check on the owner's thread for your Display as there are other two processor players out there which may be causing the same problem, and some other owner may have found the magic change to fix it.
Bob and others who have responded. The dropouts appear to be totally random to me. Sometimes they happen early in a disc and repeat, sometimes they occur near the end and sometimes they don't occur at all.
I've tried playing some discs that I have made myself from home video and there are no dropouts. If I am understanding how HDCP works I guess that this indicates you and others are correct that it is a handshake problem when playing a protected disc. I recognize that I have an "old" TV but it delivers a picture that very few current models can beat so I'll be keeping it for the time being.
Doing all the various tests of setup changes is very time consuming but I will continue trying all the suggestions that you and others have made and report my results when done.
Thanks to all for your suggestions, it's too bad that OPPO was not willing to take the time that you guys have.