UN55MU9000 client says she had multiple calls with samsung (including a dropped call after the tech heard the issue over the phone), has done a reset but problem persists.

Problem exists on various (samsung smart tv) streaming services as well as from dish stb

I heard occasional muffling almost like audio was under water - bass heavy. I did not hear any dropouts.

client says problem will sometimes get progressively bad until only unplugging tv will resolve it. turning tv off/on will sometimes resolve it.

audio was set to dolby - i changed to PCM but did not immediately resolve - probably should have powered off set and tried again in restrospect in case PCM setting not take effect until restart (?) but didn't.

Likely will loan client demo playbar to check if problem exists on toslink.

seems to me TV needs repair. thoughts?

ASIDE - called samsung CI support. spoke to 3 people who told me they would get me to the correct department. gave up.

I assume this is using ARC? What's it feeding into?

I assume this is not a commercial installation where stereo analog audio is mixed together to mono with a Y connector. Right?

its just the TV's internal speakers.

so it distorts with internal apps and an external source?  I'd say it's time to service or replace that TV.

Crucial information.
Various things sound bad. In every case, the TV and the TV's speakers are the common detail.

Yes, it's just the TV, then, if it's "just the TV's internal speakers" plus all the other stuff you mentioned.