In some earlier versions of Telescope the volume control of the video preview in the Video Manager didn't work in some browsers, specifically in Chrome and Firefox on Mac, but it works in IE, Safari, and Windows Firefox. The volume control slider wouldn't move as was expected for it to be rendering the control unusable.
This issue was fixed with Telescope Executive 2020.1. It is recommended to upgrade to this or a later version. If your TeleScope installation has customizations or you're an OnDemand customer, please contact your Account Manager or support for further assistance with upgrading.
Once Telescope is upgraded, you should be able to change the volume of the video in the Video Manager for the video previews.