Well, while MPV might be outdated, I already mentioned that I was able to get YouTube videos working by downloading a newer version of yt-dlp and creating a conf file for MPV that links to it. While I was looking into the problem, the versions available in apt for all three of these are outdated by at least a year, possible 3 years for MPV. I'm not sure if this is just a Linux Mint issue but I have noticed that a lot of the software both preinstalled and available in it's repository, are pretty outdated.
I might try downloading the newer versions of these later. I got MPV working and that's good enough for me right now.
Mint is known to use old software in its repositories as it's based on Ubuntu LTS. The flatpak mpv should work though. flatpak install flathub io.mpv.Mpv and then run it with flatpak run io.mpv.Mpv https://www.youtube.com/watch?v=dQw4w9WgXcQ
If you don't want to type flatpak run io.mpv.Mpv all the time, you can create an alias in your ~/.bashrc file. For example: alias play='flatpak run io.mpv.Mpv'. (After editing your bashrc file, run: source ~/.bashrc to activate the change). Then you can run it with play https://www.youtube.com/watch?v=dQw4w9WgXcQ.
I know it's been a while but I tried the flatpak version of mpv and it's not working. If I open the flatpak version without any music it opens but it becomes unresponsive as soon as I try to load any music. I also have no idea how to diagnose this issue as running the flatpak version in the terminal gives no error messages.
The version that comes with Linux Mint works perfectly fine, so I don't need the flatpak version, I'm just curious as to why the flatpak version doesn't work.
I think I also had this issue using Cinnamon once, but then I just used VLC instead. Never bothered to look into why. Worked fine in GNOME for me though.
If the issue is with the DE, I'm actually using the Xfce edition of Linux Mint. I would just use VLC but it gives me performance issues because I don't have the best hardware and mpv seems to work much more efficiently. But yeah, changing the audio output fixed this issue, I'm guessing the flatpak version of mpv defaults to pipewire. I was curious and I did test pipewire with both versions and I got the same results that they did.
I might try that later because, like I said earlier, I got the version of MPV that's already installed to play YouTube videos and that fine for me. I also made configurations to the preinstalled version and I don't know if it would be easy, or even possible, to transfer them to the flatpak version.
For the flatpak version the mpv.conf file can be created in ~/.var/app/io.mpv.Mpv/config/mpv/. You probably don't need to link it to the yt-dlp python file as the flatpak mpv version should come with the latest yt-dlp.