If you're in trouble with YouTube video no audio, you are not alone. Here are the possible reason below:

1: The sound is turned off
Check if any sound functions have been activated on the computer. If not, to ensure that you can hear YouTube video, try the following:
(1). Adjust the computer and speaker volume.
(2). Modulate the YouTube built-in voice controller at the bottom right of the video player.
(3). Make sure that other video players such as VLC, KMPlayer, Quicktime, Real Player or Windows Media player have been volumed up, as this might affect the YouTube.
(4). Shut down and restart the computer after the operations.

2: Hardware failure, sound card problem
Please ensure you have a sound card in your machine, and you've installed sound card driver.

3: Adobe Flash Player is too old (compatible issue)
Upgrade to the latest release of Adobe Flash Player

4: Copyright restriction
Some audio contained in YouTube video is copyrighted so that it cannot be heard. The only solution is to purchase it.

5: YouTube is in the code update phase
If you get the notification saying YouTube is in the code update phase, all you can do is waiting for the official repair.

6: Poor network
Poor network may cause the nonfluency and missing sound. You can batch download videos locally and then play.

7: YouTube audio and video separation technology
It may cause some YouTube videos have no sound or audio and video are not synchronized. Just download it.

Very useful suggestions.