Have you tried opening the sound settings on your system?

Thanks for your reply. I did exactly that. I pressed F2 and it unmuted. Yay!!! However I cannot adjust the volume (it's at 100% now). I cannot see any slider or the mixer option. I re-installed Linux and since then I see that a lot of stuff is missing, such as the little boxes you can put a checkmark in to select it. I think I need to install something (or re-install) that will allow these boxes to come back. Do you have any suggestions?

Can we see your settings? Are you running Gnome or KDE. see screenshot

You probably need to switch to another window and icon theme if you have a problem with the theme you currently use and widgets are missing or have visibility issues.

I am using Linux - Chalet OS. Everything worked well before, except that Firefox kept crashing. I reinstalled everything and now I have issues. Prior to installation I had a volume box similar to the one in your screenshot (left side). I think something didn't get installed, but I don't know what!!

On my keyboard: f1 mute f2 lower f3 higher

On my keyboard its F2 mute, F3 lower and F4 higher....but they don't work! (not on Youtube anyway) Are they only for certain things?

See if this page helps.

https://www.unixmen.com/2012003-howto-resolve-nosound-problem-on-ubuntu/

Thanks! I need all the help I can get!!! Hope it works.

No..didn't help. I don't really want to install stuff unless I am 100% sure of what it is that I am installing.