I have devil Raumfeld soundbar/sounddeck connected to my TV via HDMI. However, when I start apps like Netflix or Amazon Prime via the TV menu, I do not hear any sound.

In the TV menu, set the TV sound output to PCM (stereo) mode. If your TV does not offer the PCM (stereo/downmix) option, connect the soundbar to an analogue TV output (e.g. headphone output) as an alternative.