This is the worst nightmare as most of the time I listen to music using youtube where you can find tonnes of music ranging from oldiest to 21’s century when using Firefox on Ubuntu can view the video but no sound. Luckily it was just a missing plugin that can be install to get back the sound else will be just wipe off back to windows :(.
Strange thing is you can listen to music using the music player but just aint got sound on firefox, so if you too having the same problem don’t wait just run the command below and you’re on your way to get back the sound:-
sudo apt-get install flashplugin-nonfree-extrasound
After install the package, you need to close the Firefox and re-open back. Previously this packages was named libflashsupport and you can’t install that to the current version 9.0.4.