Try refreshing your internet browser. You may need to log out of the Mastery site and clear all your browser's cache and cookies (usually under options, preferences, or settings), then log back in and try it again.
Also try different internet browsers such as Firefox or Google Chrome.
You may need to close all open internet browsers, turn your computer off and then back on to reboot, then log back in the Mastery site and see if this solves the issue.
Also can you try checking at the bottom of the video window where there is a volume control symbol located in the lower right of the bar. If it has an x by the volume control symbol the sound is in mute and you will not hear sound from the video. Click on the volume control symbol and that should switch it back on to hear sound so it is un-muted.