Sometimes the volume on your computer, speakers or the video player may have gotten turned down.
Try these steps:
1) If your keyboard has volume buttons, be sure the "Mute" button is not on and press the volume up button several times. You can alternately adjust these in your system settings if you do not have physical buttons.
Windows: To open Volume Control, click Start, point to All Programs, point to Accessories, point to Entertainment, and then click Volume Control.
2) Check that your physical, standalone speakers are turned on, plugged in and volume up if you have speakers separate from your computer.
3) On the bottom of the video player screen - to the right of the timeline bar that shows how much of the video you have seen - is a series of vertical lines which indicate the video player volume. Click on the tallest bar to be sure the volume us turned all the way up. (See image below)
To be sure it is your computer hardware, you can try viewing the videos on another system after you have tried these steps.