Hi Reto Steiner,
I am able to reproduce the same thing, no sound.
However I have never used such type of files before, so I would like to know that whether it used to play the sound of such format files ?
When was the last time you were able to so that ?
It has never worked with Lightroom, although it should according to Adobe because Lightroom is compatible with AVCDH files. – I never have problems with Adobe Premiere.
Adobe Premier Pro is completely meant for videos and it understand many more codecs as compared to Lightroom.
I need to check that whether Lightroom has that codec that is necessary for such time of file format.