There are a lot of videos on Facebook and many of those are ads. Facebook has now decided that your ad begin without audio so that you don't annoy the viewer or those around her, so it now encourages that you design your ads with the sound off.
It's been found that 80% of people react negatively when a video ad pops up and blasts the sound, and that reflects not only on the ad, but Facebook as well. That's why it's now incorporated new tools to help advertisers out with their silent ads.
It now offers a new automated caption tool for videos to help give context to an ad that's scrolling in your feed without sound. What's more, the caption tool learns and becomes more accurate over time, although the advertiser has the ability to review and edit the captions before posting.
The problem is that a study found that 41% of the video ads on Facebook were meaningless without sound, that's why it's now encouraging advertisers to rethink their ads to work minus the audio.
You may be advertising a gig or a new release, so keep this in mind before you post.
Here's more from Facebook that explains how the new capturing feature works.