Does Video Background have a mute / sound button?
As the primary goal of the app is configuring background videos, the app by default does not show any volume controls.
However, you can go to the video configuration page to enable a mute / sound icon:
The icon works in the same way as the YouTube sound icon, so this is how the icon will be displayed in the video when sound is playing:
This is how the icon is displayed in the video when sound is not playing:
Please note that this preference is independent from the Mute option, which controls if the video sound is played when the video is loaded: