I installed the ZDF Mediathek Add-on (V5.0.2) and the ARD Mediathek Add-on (V6.0.0) on a fresh install of LibreELEC-RPi4.arm-9.95.5. Nothing else is installed, and the only settings I changed were for screen resolution (4K, 30Hz) and adjustment of the screen size. The sound I get when playing any content from the ZDF Mediathek is out of sync, whereas ARD Mediathek works (as far as I observed).
I run the ZDF mediathek Add-on (V5.0.2) on a desktop (Debian Buster) with Kodi 19.1 (from Buster backports) without any sound problems, and when running the plugin on my previous install of LibreELEC 9.2.6 on the same RPi4, sound was in sync, too.
Would there be any hints in the log files that could indicate the reason of the problem?