Facebook doesn't have audio settings and you only have the option to get the default system microphone, so you have to set Voicemod Virtual microphone as your default system microphone:
- Go to Control Panel > Sound > Recording Tab
- Select the Microphone Voicemod Virtual Audio Device and set it as the default device
- Apply the changes and it is done.
Now you can go again to Facebook and select the default device as your audio input.
* Remember, when you don't want to use the Voicemod app to transform your voice in your communication audio chat or game, you have to set your real microphone as your input device again.