[freaktm]

It's present on TV Channels with AC3 5.1 audio, but only when the Pi is set to Passthrough in audio settings.

Try setting:
"sync playback to display" to "enabled" and "A/V sync method" to "adjust PLL" and enable passthrough
or
"sync playback to display" to "enabled" and "A/V sync method" to "resample audio" and disable passthrough
or
"sync playback to display" to "disabled" and enable passthrough

and report back which of these help.

Thanks for the fast response.

The 2 last ones work properly with the sound (didn't take too much time checking image) - what's difference between passthrough enabled/disabled? Other than my receiver saying either "Dolby Digital" or "PCM bla bla" ? Is it easier on the Pi to passthrough?