See this article -> What to do if Firefox won't play any sounds

IF it still doesn't work, then Start Firefox in -> Diagnose Firefox issues using Troubleshoot Mode to check if your add-ons/extensions are causing the problems.

• Also see this article -> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working.

Check if any third-party software is set to automatically adjust volume. For instance, Skype has a setting to automatically turn the music down when a call is incoming (it's just a check box for "automatically adjust speakers volume"). And occasionally it gets stuck after a call, and keeps dropping the volume from time to time (I had the exact same issue as yours, and this seems to have fixed it). Some other programs, especially SIP can do the same, and Windows even has a global setting for that - it's in Control Panel - Sound - the fourth tab there.