Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.

https://support.mozilla.com/en-US/kb/No+sound+in+Firefox

Thanks, but I had already tried this to no avail

Thanks but I had already read that article and the suggested solutions did not solve the problem.

Thanks to those who offered a solution. I finally found the answer and here are the steps to solve the no audio problem.

1- Make sure that QuickTime is up-to-date and working properly 2- Open a URL running audio 3- Open the Volume Mixer (Speaker icon on bottom right corner on screen) 4- Make sure none of the choices are muted and that volume is set at a reasonable level.