no sound on YouTube videos

In some cases, you have to turn on the sound in the Video itself.

Sound issues may also be caused by the Web browser you are using. ... Restarting your browser and reloading the YouTube video is a quick way of solving the sound problem. However, if audio issues persist, clearing your browsing history and cache can also make a difference.

This link might also help you

https://www.youtube.com/watch?v=w5Tdsk0X5yU

In addition to checking that the volume is turned up in the videos as was said already: if the issue persists see if it also occurs in a different user account. Did you migrate everything from an older mac?