In Firefox and Chrome videos will play for a moment then stop (Ubuntu running in VirtualBox)

I'm running Ubuntu in VirtualBox and it's working perfectly in almost every single way, except when trying to play videos in browsers. I get the behaviour as you can see in the recording below.

https://imgur.com/a/nHbOgSk

The recording is in Firefox but it happens in Chrome, too. Firefox does have some extensions, but Chrome is completely vanilla with nothing extra installed.

It's not just limited to YouTube, if I view any sort of video be it YouTube, Reddit, BBC News, etc, it will do the same - play for a moment, then stop.

If I drag the playback position indicator of a video, it will load the relevant sections of the video as I move it, but when I let go it will have the same broken behaviour.

Any ideas?

Thanks!