Worse sound Internet than TV?

For a few weeks I have the problem that, as soon as I look on my TV (GRUNDIG 32 GFB 6629 Hamburg) something from the Internet (ie via Netflix or YouTube app on the TV), the sound quality is very bad (very flat and quiet ). The connection is still good (all over the same Wi-Fi network as before, no delays) and the sound settings affect the TV, no matter what. And if I watch normal TV channels, the sound is okay again, so it's not the sound settings. What could it be?

Not everything new is better than the tried and tested one!

I have been wondering for some time whether it is worth striving for every new trend.