We've tested the app you mention – it is not actually adjusting the input gain as we suspected and as Apple also confirms. If you record at high levels, you will hear distortion as a result of clipping. Deceptively, they are simply lowering the amplitude (volume level) after the file is recorded – the app is NOT affecting the actual input gain level. You can see this for yourself by recording a high input level and viewing the waveform in an audio editor, which we just did. You will notice that the waveform is squared off – regardless of whether their "input" level is adjusted to give you the impression that it's affecting the recording. We could certainly add this, but many users – especially professional ones – would quickly complain when they would ultimately realize that – regardless of playback level – the files are in fact clipped. You can get the same results with iProRecorder simply by lowering the playback volume, or lowering the waveform gain in an audio editor such as Peak – in either case, the output level will appear to be fine, but the signal will still be distorted as the clipping happens at the analog to digital conversion stage.
Here is a direct quote from Apple regarding this:
Subject: Re: Set Volume for Recorder Queue
On Mac and Desktops, there are APIs to set volume controls, etc on audio devices - these are found in the <CoreAudio/AudioHardware.h> header.. The "Audio system pref pane" (Sound Prefs) or Audio MIDI Setup, use these APIs to do this
On iPhone there is no API to control the volume of the audio input.
The only true solution available for controlling the input level on the current iPhone, is to use additional hardware such as the Blue Mikey or Alesis ProTrack. I hope this clarifies things.