Smartphones keep getting better at capturing video. In order to record better audio, you will need an external microphone though. The Sennheiser Memory Mic can help. It is an omnidirectional condenser microphone for iOS and Android smartphones. It can record and store up to 4 hours. With one-touch audio/touch sync, you don’t even have to maintain the connection between your phone and microphone all the time.
The Memory Mic comes with a magnetic clip, so you can wear it easily. It has a frequency response of 100 Hz to 20 kHz. You can order yours for under $200 at this time.