Hi Inix,
before you spend some Money on a HiFiBerry I suggest you should try the following:
1. Your Nubert Speaker have an ARC-HDMI Port.
On Page 9 in the German Manual to your Speakers it says that CEC and HDMI ARC on your TV must be enabled to made the HDMI-Port of the speakers work.
The Raspberry didn't have an HDMI-ARC Port, but did you enable the CEC-Funktion in LibreELEC/Kodi ?
I don't use CEC but as far as I know you can find the Setttings under "System" -> "Input" -> "Peripherals"
2. Did you use the right HDMI-Cable ?
Don't use any Adaptor from Micro-HDMI to HDMI - Use a cable like this (from your screenshot I assume that you are from Germany or another German speaking County, so this is a Link to a German Webpage)
offizielles Raspberry Pi 4 Micro HDMI Kabel, schwarz, 2,0m | offizielles Raspberry Pi 4 Micro HDMI Kabel, schwarz, 2,0m | HDMI Kabel & Adapter | Kabel & Adapter | Raspberry Pi Computer | Raspberry Pi | BerryBase
I know you did this before, but please try again:
3. Connect the HDMI-Port right beside the USB-(Power)-Port (HDMI 0?) to your TV
4. Connect the other HDMI-Port (HDMI 1?) to your Nubert Speakers.
5. This is very important:
a) First turn on your Nubert-Speakers
b) Than turn on your Raspberry Pi4
c) And at last turn on your TV
6. Now check if you can select your Speakers in LibreELEC/Kodi
7. If you can't get a Picture on your TV or can't select your Speakers turn off the Raspberry, the TV and the Speakers
8. Switch the HDMI-Cables - The Port beside the USB-(Power)-Port to your Nubert-Speakers and the other one to the TV.
9. Now go back to Point 5 & 6 above
10. If this doesn't work either why not try an older Version (9.2.8) of LibreElec ?`
Yon can find the Download here:
LibreELEC Raspberry - LibreELEC
11. If this doesn't work too you can still buy a HiFiBerry
Oh, and by the way:
The Nubert "nuBox" A-125 is no "Soundbar".
It's only a pair of Stereo-Speakers with an HDMI-Input.
Look here (sorry it's in German)
Aktivlautsprecher nuBox A-125 von Nubert
They only work with a "PCM" Stereo Signal (Highest level is 24 bit / 192 kHz)
(You can read about that in the Manual)
They can't handle things like Dolby Digital, Dolby True HD DTS or Atmos.
So in the Kodi "Audio Settings" you have to disable "Passthrough" and set "Number of Channels" to "2"