If you head over to your account settings in WhatsApp, you'll see a new Two-step verification option. Tap it to get to a screen that explains what 2-step verification is and get the option to enable it.
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You'll then have to enter a 6-digit passcode that you'll be asked for each time you try to register your phone number with WhatsApp, and confirm it. You'll also need to give an email address that'll be used to reset your passcode should you forget it.
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Once that's done, two-step verification will be active on your phone number. No one will be able to activate WhatsApp with the same phone number unless they have the passcode or access to the email account to reset it.
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Another welcome improvement to WhatsApp, which was introduced a few beta versions ago too, is the option to play audio messages in the background. Previously, you had to keep WhatsApp open and stay within the conversation to hear any voice message received in it. Now, you can wander away freely, switch to other conversations or even other apps, and as long as the voice message isn't over and your phone's screen doesn't turn off, the audio will continue playing.


So I can finally listen to my aunt's 3-4 minute ramblings without hating myself each time I get tempted to move away from our conversation window. The joy!
There was also a point where WhatsApp made it possible to stream shared videos before/without downloading them, but that option either isn't live for everyone or has been taken off in the most recent beta because I can't see it on my phone.
Both two-step verification and background audio message playback are available in the latest WhatsApp beta (and a few versions prior) that you can grab from the Play Store by joining the official program or from APK Mirror.