My experiences are a bit different than those above, so I'll append them here.
With Audio = Auto Select and HDMI = Auto
Stream #0:1(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1(side), fltp, 448 kb/s (default) - nothing but clicks/pops/white noise
Stream #0:1(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, stereo, fltp, 384 kb/s (default) - nothing but clicks/pops/white noise
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, various kb/s (default) - plays fine in all cases with no dropouts
With one TV, which is able to accept DD as input, changing HDMI to DD results in all AC3 files playing fine with no dropouts. On a second TV, which cannot decode DD, there is no setting of HDMI that produces listenable audio for AC3 encoded files.
All files played fine under OS 9.4.
Model: Streaming Stick+ 3810X
Software 10.0.0 build 4198.50
Issue ID: 83-218-726
RMP 5.5 Build 9
File source: DLNA
File wrapper: MP4
A one-minute video of the file playback noise was provided to first level support a few days ago at their request.