Thanks for your post. If I understood you correctly you want to play the sound of the background video when it is auto-played!?
I am afraid this can’t happen programmatically, since browsers will block the entire video, as playing a sound without user’s consent (a click or a tap) is a violation.
Please read this for more info: https://developers.google.com/web/updates/2017/09/autoplay-policy-changes
Hope this answers your question!
Best of luck,