It took a while but now it is perfectly solved.
So if you want to transfer the BPM e.g. out of YouTube Video Stream from your Smart-TV to DasLight4 it works like this:
Take the optical cable (Toslink) coming out of the TV and connect to this hardware device:
"Delock Konverter Digital Audio to Analog HD"
And you need an external USB sound card for your Mac / PC "UGREEN Externe USB Soundkarte Klinke USB Adapter for Computer, PS5, PS4, USB Audio Stereo Adapter External Sound Card".
Now connect via TRS headphone cable to the USB soundcard "microphone in" port.
Now MacBook or PC is recognizing the incoming audio stream as a microphone. Finally select within Daslight "BPM" and switch to the UGREEN USB Microphone. Now the BPM are perfectly recognized within Daslight in real-time.
DasLight4 is really a great software turning lightning dreams into reality. Thank you so much for it.