WhatsApp working to bring new sharing-related feature to mobile users
The new feature spotted by WaBetaInfo can be enabled by tapping the volume toggle that pops up once you start editing the video before sharing it. If you're already on the correct version number and you still don't see the option to mute videos before sharing them, then you should wait a bit since WhatsApp is rolling out the feature in waves.
As far as the general public goes, it's unclear when exactly the new feature will be ready for prime time. Usually, testing features before they're made available to everyone takes time, so it's hard to say when users, other than those enrolled in the beta, will have access to it.
The new beta version of available for download via the Google Play Store, but keep in mind that it might not be as optimized as the stable version that everyone uses.