The standalone application mutes the input by default to avoid loud feedback in case the I/O is not set up correctly.
Does your app show a yellow bar on top like this?
By pressing the 'Settings' button, you can set up your input, and output, and press the 'mute' button into the off position.
For Mac users:
If that does not help, and you are on Mac OS, it is possible that the system has blocked microphone access to the application.
Here is how to enable it:
1. On your Mac, choose Apple menu > System Preferences.
2. Click Security & Privacy.
3. Choose the Privacy tab.
4. Select Microphone.
5. Check the box next to the AmpKnob to allow it to access your microphone.