[Tarkus_]

Have you tried it with your Firestick set to Stereo?  For some reason, surround in Kodi works way better on my system with it on that setting.  It's often the only way I get sound out of my rear speakers, even with 5.1 content.  I don't know if it's something weird with my surround system or what.

Oh, wow.. it's weirldy works

I'm using Kodi 18.9 right now, passthrough option gone with Firestick set to Stereo. But yeah, it gives me Multichannel PCM on everything.

No surround sound on other streaming apps (netflix, etc) though, obviously.

Do you guys mean setting it to 2.0 channels and disabling 'allow passthrough'?

Correct.  It's much more reliable (at least for me) to allow Kodi to decode the streams than to try and get them passed through to my surround system.