for some strange reason none of my video is playing sound
I see from the screencast that you are using a fairly old version. You might have accidentally attached a volume or mute filter to the whole timeline. Click the top left corner of the timeline (blank block just below menu button) to select it, and look at Filters to see if there is something. In newer versions, that block has a label (“Master” in English) will show a little filters icon if it has filters attached to it.