I have the exact same Kubuntu virtual machines set up on a Windows host and a Mac host in VirtualBox.
The VMs under the Mac can play web media right out of the box. The VMs under the Windows 10 host cannot play any media on Youtube, Soundcloud, etc.
I've tried installing different distros under the Windows host and I've tried installing all codecs, including Pepper Flash, Ubuntu restricted extras and Mint meta codecs. Nothing changes the fact that the Kubuntu and Mint VMs under a Windows host cannot play media, whereas the VMs under a Mac host work out of the box. I am puzzled why an identical installation on two hosts works differently.
I've requested HTML5 in Youtube but that doesn't load the videos either.