If you’re not happy with the quality of the iPhone internal microphones you could try this. A USB microphone plugged into a camera connection kit and then plugged into the iPhone (or iPad). It gives a much rounder tone than the internal mic. The one I’ve been using is a rather studio-looking Samson Meteor (around £50 on Amazon) but others – some of them standard-looking handheld mics – are available.
If you want to really hear the difference have a listen on headphones to this: