Audio Interfaces and Microphones
The nature of the iPad’s audio socket is often not understood in detail, since it’s almost exclusively used as a headphone or stereo audio output using a three-connector (TRS) 3.5 mm plug. On the original iPad, a built-in microphone was located immediately next to this socket, so it wasn’t generally understood that the socket is in fact wired TRRS to support four connections, of which the last is a mono audio input intended for an alternative microphone. See the Appendices for more details.
The audio input is intended primarily for connection of an alternative microphone, but with a suitably wired source cable should also allow appropriately written apps to record audio from sources such as CD players ...