Does Video Background have a mute/sound button?
As the primary goal of the app is configuring background videos, the app does not have any built-in volume controls.
On the other hand, most users know how to control the volume of their devices and don't expect to have to look for a button to do that in a web page.
You can enable or disable the sound for a video on the app preferences page, but it won't be manageable from the public page where the video is displayed.
To change the video sound configuration go to the 'Config' option in the app preferences page:
Once you hit the configuration button, scroll to the 'Optional Preferences' section. There you'll have the option to display the video with sound or you can check the option to mute the video.