Cost: free version or a 4.99 option
For on the spot audio interviews or to capture some natural sound, Recorder Plus II is a good option. Recorder Plus will allow you to take some quick audio, edit it, and publish it immediately. In this app you can record audio and edit the audio with the touch of your finger. The input levels can be adjusted so if the noise level is too low, adjust the input to capture the sound better.
Recorder Plus will also work with other apps on your device. If you are using iMovie to put together an in the moment movie, use Recorder Plus to edit your audio then export it directly to iMovie. You can also export the audio to Evernote or as a file to email for later use.