I had a question come up today while talking to another Creator today:
I want my videos to autoplay with sound, and it's not. Is there a way to do that?
It was a good question! We have to write a full article about it for our Help Desk but the short answer is:
Because most web browsers are against it! To best adhere to the user experience, they made it standard practice that if a video automatically plays on a site, it must have no audio track or be muted. User action, like clicking a play button, is required to play a video with sound.
Browsers, such as Safari and Chrome (one of the two most popular), have specified this in their own FAQs- which you can learn about here:
It's also a benefit because autoplaying sound- especially on a loop- can drain your battery quicker. 
If you've been wondering about this, I hope this helps.