The video player was upgraded recently - it is a third party player and while embedded on the site is held elsewhere. The issue with the audio has been reported to the provider and they are working on a fix. This is not specific to this site and definitely nothing to do with Windows 11 - I use linux as well as Windows and both have the same issue. If you have ever used StackSkills, they use the same player and it also has the same issue too.
Again, key controls, that would be up to the third party provider to implement. Space does toggle play and stop so they do have some keyboard functionality but I could not say what.
The clicking the progress bar of the video does not pause the video if you click on it, but if you’re off by a tiny bit it does - that is annoying and there used to be a bit more tolerance.
Sound at 50% I’ve never experienced, at least for a long time. Which browser are you using? I personally use edge (even on Linux) but I’ll have a look in firefox - I do not use chrome…ever. It possibly could be a cookie issue.
You have to remember, Youtube has been around for a very long time (2005) and also these days is so full of ads it’s virtually unusable - no ads here.
I hope this helps answer your questions/helps you understand what is going on, at least with the sound issue. I hope that gets resolved soon too.