Hello !
I have the same issue, i contacted unbounce support and they said they were working on it. But chrome said they would block autoplay videos so this isn’t really a surprise, i doubt a real fix can be found
Chrome only blocks autoplay videos that are not muted, and this started in April 2018, these pages just stopped more recently than that. In their docs here https://developers.google.com/web/updates/2017/09/autoplay-policy-changes#mei - it says “Muted autoplay is always allowed.” these are muted as they are background videos. Also in this article on The Verge - https://www.theverge.com/2018/5/3/17251104/google-chrome-66-autoplay-sound-videos-mute - I don’t understand. I’ve also seen background videos working on other sites I’ve built. They also play automatically in Unbounce preview. They all work in Chrome Canary, which hopefully means that the newer versions of Chrome won’t do this, or Unbounce will fix as they say.
And they seem to work now, so it’s been fixed! By either Unbounce or Chrome, not sure which. One thing I noticed is that you have to re-publish the page to get it to work - so I’m guessing it’s something Unbounce fixed.