Crackling and signal loss when streaming 5.1 material

[SANAFABICH]

So this is my set up:

Roku Stick or Fire TV Stick connected to my LG TV. Audio goes out from the TV via a toslink optical cable to my rather ancient LG AIO 5.1 system, which only supports DTS and Dolby Digital.

And this my problem:

When streaming 5.1 movies on Netflix or Prime Video, I do get 5.1 sound but I also get occasional crackling and loud pop noises. Sometimes I get a couple of seconds of silence (loss of signal?); sometimes it never comes back and I have to change from optical to TV speakers, and go back to optical for it to return. As mentioned, this only happens when the movie has a 5.1 sound track.

Now, I know this AIO HT is quite old, and I do plan on buying a new one. But I want to know! Why is this happening? At first I thought it was the Dolby Digital Plus / Atmos track that streaming services use, which my HT isn't compatible with. I don't have this problem with Blu-Rays (player is connected via a coaxial cable) that have a DD+ track, but I thought maybe streaming services use a different kind of DD+?

I had a Roku Stick, so I bought a Fire TV Stick 4K just for this reason, since this stick does decode DD+ into regular DD, AFAIK. But I'm still getting the same crackling noises even when the Fire TV is set to output DD.

So what is it that's causing this problem? Is it the optical cable? Maybe it's faulty. Is it DD via optical cable that isn't fully supported? Is it my TV? I'm drawing a blank here, and I thought maybe you guys could help me solve this mystery!