Once you start recording, a mic icon is visible in the controls. Click on the icon to mute or unmute your video.

 

mute or unmute video

 

P.S: If you choose to hide your controls, use the shortcut keys to mute/unmute a video. Take a look at the Shortcuts here: https://storyxpress.co/help/faq/record-upload-edit-faq/recorder-shortcut-keys/.