I don't know if this will help anybody else, but what finally worked for me ("worked" is a relative term, here) is to set the Audio HDMI settings to Dolby. Not Dolby+ and not Autodetect. Just Dolby.
True in some cases. It's my experience that this solution works only if the receiving device is capable of decoding the Dolby Digital signal. If the receiving device isn't, then it does not work, and setting HDMI to "PCM Stereo" does not appear to work, either.
Keep in mind that for passthrough-only models (those lacking a Dolby decoder), setting it to anything other than DD+ (auto or manual) will limit the output for NetFlix/Prime/Hulu/D+/etc to PCM 2.0: these apps exclusively use DD+ and/or rely on system configuration for audio stream selection/availability.
(For those models with Dolby decoders (e.g. 4640 or 4800, or TVs), manually setting it to DD transcodes the DD+ to DD, so it isnt a problem)
So, while this may provide a fix/workaround for UPnP/DLNA streaming in some cases, it will limit output to PCM 2.0 for all of the DD+-only apps for passthrough-only devices.