Thank you owlhugin,
but it is an old, well known and from many users acknowledged bug.
It happens only if you use movies with the most popular frame-rate of 23.976 Hz and an AV receiver and pass-through is enabled for all supported audio formats in Kodi. (This is the standard use of all homecinema enthusiasts. For a direct connection to a stereo TV set I recommend to buy a much cheaper box.)
Himedia/Rockchip has to do something, the customers as Himedia Beta-Testers already spend a lot of their valuable leisure time to test a lot of possible configurations and movies because Himedia is not willing to recommend good settings for different needs or to make reasonable default setting in Custom-Kodi.