[Etzi]

Yes. But I don't really get this approach. Why doing such a weird settings combination that affects everybody instead of doing a setting in advancedsettings.xml that is straightforward and well understandable?

Suggested that approach to @fritsch at some point, but it required some hacks and they decided not to go that route AFAIK.
He may comment this in more detail, if he sees fit.
Anyway at least we have somekind of working solution for multichannel so, I would not like to be too picky about its actual implementation.

What if I want to sync to display and use AC3 transcoding to get synchronized PT audio? Am I always stuck with those 384kbps, no matter what my device is? I think I am missing something because this for sure wouldn't be a good solution...

Yes you are stuck, as we all are if you have Android, as there seems to be more broken devices than actual working ones.

PS: Believe me, but you won't hear much difference between 384kbit/s and 640kbit/s...