Hello Varlok1...

I think media autoplay was marked as false.

for allowing media autoplay , please follow these steps:

1. Click the menu button and select Preferences. 2. Select the Privacy & Security panel. 3. Scroll down to the Permissions section. 4. Find Autoplay and click the Setting button. 5. Use the Default for all websites drop-down menu to select how you want Firefox to handle media autoplay for all websites.

• The default Block Audio setting will prevent autoplay for all media with sound.
• The Block Audio and Video setting will prevent autoplay for all media, including video, even though audio is muted.
• The Allow Audio and Video setting will allow all media to autoplay.

6. After you have made your selection, click save change.

Hello,,

Open Firefox and play content that should play sound, such as this Firefox video or a Flash video with sound.

   Click the volume icon in the Windows taskbar.
   Click Mixer. The Volume Mixer window will appear.
   Make sure the sliders for Mozilla Firefox and Plugin Container for Firefox (for plugin content such as Adobe Flash) are not muted or at the bottom. 

Win10 Volume Mixer

Check your audio device

Make sure that the audio device (such as speakers or headphones) are turned on and connected to the computer properly. Other solutions

Check out whether other applications on your computer can play sounds. If they can't:

   Follow Microsoft's Fix sound problems tutorial. 

If Firefox is the only application that you can't hear sound in:

   Try the solutions in Fix common audio and video issues.