Thank you for your message, I would be happy to offer you some insight into this. It’s worth noting, that it’s not always advisable to have auto-playing videos on your website. For some context, starting in April 2018 one of the most popular browsers, Google Chrome, by default disabled autoplay on videos that have sound. This article explains more about that change. Even without that change, it’s not reasonable to expect that the autoplay will work on all devices and browsers. As this article from a Shopify Expert note. There are a lot of ways for people to disable autoplay in their devices/browsers.
Autoplay videos can also have a negative impact on accessibility, as this can cause discomfort, or even pain, for some users who are sensitive to movement or sound. By ensuring that the customer has control over the video playback, a site is more accessible to a wider audience. With that said, if you are using a third-party theme, I would recommend reaching out to the developer of your theme to look into this possibility. Alternatively, you could always reach out to a Shopify Expert to look into this possibility.
Thanks again for reaching out and feel free to reply back here if you have any questions!