Hugo E. most likely because its a background video and those tend to not have sound for a host of reasons.
In the future, if you have any questions around Webflow - be sure to paste in your Webflow read-only link, this allows anyone to see the structure of your site and be able to help more effectively!
Hugo E. my guess is that you'd have to use a video hosting service like Vimeo, YouTube, or Wistia as Webflow doesn't want to take on the load of hosting video. You likely wouldn't want that either since Webflow also charges for Bandwidth but that is mostly dependent on traffic.
My favorite video on background videos comes from Timothy Ricks - he covers all kinds of tricks to make them work in various ways. https://www.youtube.com/watch?v=m-OQTr0akKc