First of all, to determine the reason for your YouTube sound problems, you need to take a look at the YouTube video player itself and the volume icon in particular. An "X" over that icon on the player means the sound has been muted. To increase the volume, click on the volume icon, then drag the slider to the right. Another thing to consider is your computer system’s volume. If it’s turned all the way down or your speakers or headphones are muted/switched off, you’ll hear no sound on YouTube, regardless of the volume level on the YouTube player. Therefore, make sure you check the external speakers and adjust the volume. This may sound like an obvious thing, but you’d be surprised how many times it gets overlooked.