This post is a shout out for help and suggestions. 2 days ago I found out that my MBP wasn’t able to playback mp4 anymore - or let’s say, not as usual - but wait for it.
I tweeted about it:
But it’s more complex.
The symptoms
I first saw this problem while using the photos app. While I was scrolling through the recent photos I came to a video I took with my iPhone.
I get this message whenever I try to watch a video in the photos app. Next problem is the Home app in Mojave. It’s not able to display live camera content anymore, not even the preview images on the favorites page. Another problem is with the quick preview
In short: this happened from one second to the other, but right after I updated to the latest 10.14.2 beta, so I associated it with this update. That was a mistake, the beta is not to blame. The problem still persists on a freshly installed machine, even on High Sierra.
The next test was trying to play a video stream in Safari, i.e. Netflix and Amazon Prime. No way, videos are not played back. The same is with other video content embedded in e.g. Facebook.
The machine has a dedicated NVIDIA GT650M graphics card built in, I have the driver installed and according to EtreCheck it’s loaded and working. I also have no issues with the graphics at all, the performance is great as usual. The Apple Hardware Test also doesn't give any suspicious results.
I opened a new thread in Apple’s support community: https://discussions.apple.com/message/34143353#34143353
Workaround
After I installed Chrome (which is my default browser) I did another test, and guess what: I am able to play streaming content like Netflix and Facebook videos and (this is the weirdest): I can drag mp4 files to the browser window - and it perfectly plays them back. By the way: even VLC and IINA could not play back these videos, they just crash or freeze. Quicktime gives error messages (see my Apple forum post). Quicktime files play as usual though.
Call for help
So obviously I am seeking help for a solution. I doubt a hardware issue though I am actually not sure. Software-wise there seems a way to use hardware acceleration (which I think is responsible to playback video content fluently and in general). By the way: a Windows VM in Parallels also plays back everything - via the virtualized hardware acceleration.
So if you have any ideas please let me know! 
Small update - and progress
I just found out that video playback works fine when I connect an audio device. My MBP doesn't have sound as the internal audio interface is completely dead. Once I connect headphones or other sound devices, everything works fine. So I guess I need a "dummy" audio device as a "default" one to fulfill the preview's and Quicktime Player's needs 
Confirmed
I installed "Soundflower" (again) as a dummy sound device and now I am able again to playback mp4 without having a "real" audio device connected.