When you add one or more videos to an artboard, you can mute a specific video (or all of them). Just follow these steps:

1 - On your Simplified workspace, go to Video

2 - Go to your project by clicking Projects on the left side menu, then selecting a project

3 - On the artboard, select the video you want to mute by clicking on it

4 - On the top toolbar, click on the speaker icon, then slide it all the way down to mute the video ( or just slide it slightly to reduce the video volume without muting)