I finally have a working stream on Sonos - kind of.
The solution is to choose „Raw Audio Stream“ as output container.
The stream works for about a minute with ~45% CPU usage on the Pi (3b+). Then all of a sudden the tvheadend process eats up >100% CPU, the UI gets unusable and the stream breaks. It might get back to normal after some time but then start all over again. This happens with both the latest 4.2 as well as 4.3 releases. It happens with VLC client as well as with Sonos.
I already tried to deactivate everything which is possible, so no epg crawler, no refresh, no service scans, no magic at all. Still the same behaviour. The log in the GUI doesn’t show anything suspicious. Is there another way how I can try to find the issue?