If there is no sound coming from a YouTube video you are watching, first check the volume levels of both the YouTube player and the computer to ensure neither is on mute. Also check speaker or headphone connections, if using an external device. Finally, test videos on other sites or programs to ensure that the system is functioning.
One of the most common reasons for a loss of sound on a YouTube video is accidentally muting the volume on the player or the system. To change the volume on the video itself, hover over the speaker icon on the bottom controls section and see if it has an "X" over it, indicating a mute setting. Click on the icon to unmute it, then drag the slider to increase the volume. The location of the system speaker volume varies according to the computer, though most appear within the task tray either at the top or bottom of the system. Some keyboards feature buttons dedicated to controlling the audio.
If you are using headphones or external speakers, also make sure that the device has a secure connection to the proper port. If the video still does not play sound, open another video or program to test the speakers. If the system projects sounds, but the video does not, it may be a browser issue or the video itself may not have the proper sound settings.