This is very useful if you don’t have a microphone available but need to quickly record a sketch into Logic for example. This is how to set up your iPhone as a microphone using your Mac’s built-in utilities, no third-party software needed:
Connect via cable: Use a charging cable (Lightning or USB-C) to physically connect your iPhone to your Mac.
Select “trust this computer” on the iPhone: Tap Trust on your iPhone when prompted, and enter your passcode.
Open Audio MIDI Setup: Locate and launch the application in the Utilities folder (/Applications/Utilities).
Open the “Audio Devices” window: If the window is not visible, navigate to Window > Audio Devices (or press Command + 1).
Click “enable” below the iPhone name in the sidebar.