I thought perhaps a loose cable, but I asked our in-house audio enthusiast:
"Sounds more like a decoding problem to me. With a bad (or loose) cable I’d expect to see video glitches as well, since video requires much higher bandwidth than audio, and flaky video signal would interfere with HDCP (copy protection). But it’s very easy to swap or reseat cables, so it can’t hurt to try.
This reminds me of a problem we had years ago, when the streaming services started dropping the audio bitrate to save bandwidth, and my fancy processor was having trouble dealing with the low-quality audio stream. As though the manufacturer never tested extremely low bitrate DD+ because “who would ever do such a thing”. I’ll tell you who–Netflix.
Maybe the soundbar can’t process Atmos or DTS:X very well even if it was advertised to do so. He could try disabling the spatial audio formats and just using traditional 5.1 lossless formats (DTS-HD and Dolby TrueHD). If that works consistency, then problem identified.
Maybe it’s the streaming app. Is he using the built-in apps on the LG TV or a standalone box like an AppleTV or Roku? I’ve only ever used AppleTV and it has worked well.
Maybe it’s streaming, period. Does he have any audio problems playing back a Blu-ray or UHD Blu-ray disc?
Speaking of audio quality, bluetooth has only a small fraction of the bandwidth of any lossless surround sound format. The audio being sent to those bluetooth surrounds is definitely being recompressed. Maybe that’s part of the problem.
In general I’m no fan of soundbars, and I’m certainly no fan of bluetooth for anything resembling “quality”. I’m not convinced there is much utility in decoding spatial audio formats (Atmos, DTS:X) if you don’t have height and/or ceiling. If constrained by space and/or budget, and you can’t do more than 5.1 (or possibly 5.0 in this case, I don’t know if he has a sub), I would personally build a 3.0 system. That is, front/center/left speakers, with a decent receiver and decent separate speakers. E.g, mid-range Denon or Integra receiver, maybe something like PSB Alpha speakers. And if there still is room in the budget, again for me personally, I would put any extra money into speakers. E.g. entry-level B&W or Dali. If sound quality is paramount, then speakers are the most important component IMHO."