Firefox (and Windows) is up to date, but this problem has existed for several versions now. I can't be sure when it started. This is on a Windows 10 PC. My normal routine is to leave several Windows and Tabs open. When left for a while, Windows turns off the screen. When I resume, I find that sound doesn't work. I check that the speakers are not Muted in Windows, I check that Windows Media Player doesn't have the sound switched off. I run Troubleshooter, which finds nothing wrong. I have to reboot the PC, to get the sound back.