It is possible that the security software/antivirus on your computer is quarantining the ffmpeg.dll file.
As an experiment, you can try 
TEMPORARILY disabling the security software/antivirus on your computer for a single IDE startup attempt to see if the problem goes away:
- Disable the security software/antivirus software.
- Start Arduino IDE, just as you did before when you encountered that ''
The code execution cannot proceed because fmpeg.dll was not found." error. - Wait for Arduino IDE to either start up successfully or fail to start.
- Immediately enable the security software/antivirus software again.
If the problem doesn't occur when you tried starting Arduino IDE while the security software was disabled, you will need to adjust the settings of your antivirus to put the appropriate file, folder, or process on the "allowlist" so it doesn't interfere with compilation.
Please be cautious about working with the security software disabled. This is only about temporarily disabling it for a quick test. If you don't feel comfortable doing that, fine. You can try going straight to configuring the security software so that it does not interfere with the Arduino software.