Your solution worked. I found a setting on the computer where Firefox itself was muted. I was surprised because Youtube videos gave good sound when launched in Firefox.
Perhaps you could post your solution on the answers forum. The problem is that it is difficult to find the screen where the muting settings for the browser are located. I cannot even recall now how I found it. So telling people to make this change without telling them how to make the change may be frustrating
I am glad this worked for you.
The setting is usally under the mixer option under the volume control. Some computers you can find this in the control panel or just at the lower left hand side of a computer screen where you increase the volume. Each computer might be setup differently too so that is why I indicated the way I did.
But, I am glad that you are able to hear the sound now.