When you add one or more videos to an artboard, you can change the volume of 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 change the volume of by clicking on it

4 - On the top toolbar, click on the speaker icon, then slide it up or down until you reach the desired volume