Is there a way I can use an iPhone/Android phone as an external microphone for my Mac? Ideally without a cable i.e. bluetooh/wifi.
Since this hasn't been answered yet, I would be okay with using cables too if people know how to do that.
Using a cable, your iPhone can be an audio source by using the built-in Audio MIDI setup. Plug in your phone and click enable.
Next make sure your mac is set to use the device for input. Then on your phone select audio output to Dock connector. Now any audio from your phone will be sent to your Mac. To use it as a microphone you now need to use an app that lets you feed back your mic. There are many apps that do that, I use https://itunes.apple.com/us/app/microphone-free-vonbruno/id1070812067?mt=8 since it is free and simple.