Ah, I see. That's not due to the Chromium version though, it's because the Chromium that comes with Qt releases (which is what's shipped in qutebrowser releases) is built without proprietary codec support.
Unfortunately, support for proprietary codecs like MP4 and such requires building Qt from sources, and would also require me to acquire licenses for them all (I believe they're free until a certain number of distributions, but also with all the indirect ways you can get qutebrowser, I can hardly even provide that information).
This isn't an issue on Linux, because those licenses have some sort of exception in the sense of "if shipped with an operating system or its packages".
Homebrew seems to build its Qt packages against system ffmpeg with proprietary codecs enabled, and there's an issue open which would at least allow you to build a custom build against that: https://github.com/qutebrowser/qutebrowser/issues/4127
Maybe I should look into whether I'd be allowed to redistribute such builds (or what kind of paperwork I'd need to do for it). Unfortunately there's a lot on my plate, and macOS/Windows are admittedly somewhat second-class citizens as I don't use those myself.