Sometimes, for no apparent reason, the browser turns the volume for the video all the way down or off. If that's the case you can turn it up by clicking on the bars in the lower right hand corner as shown above. More bars means louder volume.
This solves 90% of the issues with no sound, but if that doesn't fix the issue please let me know so I can troubleshoot.
Please let me know the exact title of the course, lesson and number so I can find it. (i.e. Beginner's Journey, 2nd Adventure, 2-4 Add 2nd String Notes).
Alternately, you can write a comment in the Comment Section directly below the video. Your message will take me directly to that video.