So you want to capture higher quality audio with your iPhone video? Investing in a decent professional microphone is a good idea. The Rode i-XLR is a digital XLR interface that lets you connect dynamic and self-powered microphones to your Apple iOS devices. It combines with an app to help you capture better audio.
The device offers 20dB gain for your microphone. It features zero latency headphone monitoring. Rode i-XLR is available for under $150 at this time.