I only know the sure way for this using Windows but you're OS is unspecified. So here's the best way for Windows:
1. Search 'Command Prompt' on your computer.
2. Type in (EXACTLY) "ffmpeg -version" and hit enter.
3. If it says "unknown name" or related, try installing again and make sure all the steps were done correctly.
4. When FFMPEG is installed correctly you will get lots of random text from cmd prompt.
5.Restart Minecraft.
6. You're good to go!