First type about:preferences#permissionsData into the Firefox URL bar, then press ENTER
Click Speaker
Check the sites you're having an issue with aren't listed there as not being allowed
If not, you've probably muted the Firefox app within your OS
On Windows 10 look for the speaker icon near the time, on the Taskbar
Right-click and choose "Open volume mixer"
Make sure you have the volume-slider there up for Firefox and it's not muted at the bottom
In Windows 11 you may be able to do it the same way as for Windows 10
If not, click Start then ⚙ Settings
In the System area, click Sound
Look for "Volume mixer" and click that, then adjust Firefox's volume there
