The FiLMiC Pro app is quite popular among iPhone movie makers -- a sister app will be released this year that will turn iOS devices into a field recorder in no time at all. FiLMiC Audio will be able to record WAV/AAC/PCM and AIFF at 44.1kHz, 48kHz or 96kHz. 32 bit floating point resolution is supported for recording.
FiLMiC Pro Video-App
Of course, sound should also be able to enter via an externally connected microphone and be levelled (gain control). If you work with two devices in parallel, one for filming and one for sound recording, you can easily replace the audio track of a FiLMiC Pro video with that of FiLMiC Audio using an Auto-Sync feature.
FiLMiC Audio will be released in the course of the year at an as yet undisclosed price; for orientation: FiLMiC Pro costs 17.99 Euro resp. 13,99 Euro.