After digging a little bit deeper we found a solution (at least for unmanaged devices).
Turns out that in our case Chrome is causing this issue. The new version (66) of chrome implements a “block-autoplay-video” feature. Turns out it is not that easy to turn off there is a way (at least for unmanaged devices):
In your chrome browser on the respective device go to chrome://flags/#autoplay-policy and here you can set the option to “No user gesture is required”. That will deactive the “block-autoplay” feature.