If you use a headset like the Apple Earbuds both the headhones and the microphone will be active. But if you want to use a separate microphone (e.g. plugged into the lightning port of your iOS device), you need to turn off the microphone of the headset. There is a simple trick to achieve this: Just put a regular (3.5 mm) audio extension cable between the headset and the device. Make sure the cable looks like this:

This is a so-called TRS (Tip-Ring-Sleeve) cable with 3 contacts. In contrast to the TRRS (Tip-Ring-Ring-Sleeve) cable of smartphone headsets it is missing the 4th connection for the microphone. As a result, the TRS cable will turn off the microphone of the headset, but the headphones will continue to work.