I am not aware of an embedded odoo function to unmute the video.
But you can adjust the attributes of the video using the HTML/CSS-Editor. 

Search for the "data-bg-video-src" parameter and change the mute=1 to mute=0 in the URL of the video.

Cheers
Nils

EDIT: additional infomation...
when changing the mute parameter via HTML editor, the autoplay function does not work anymore. Seem to be a known issue...
https://www.odoo.com/de_DE/forum/hilfe-1/youtube-video-can-t-autoplay-with-sound-if-you-change-it-to-mute-0-the-video-stops-working-can-someone-help-me-please-163994

an option is to use an iframe... anyway... If you want to put the video in the background, fitting fullscreen (width and height) is not that straight forward and requires some additional css styles.