There is a previously recorded sound file containing user's voice. What I need to do is apply some sound filter to change the voice and after that be able to save and play modified version.
AudioKit can open file using
[[AKFileInput alloc] initWithFilename:file1];
[[AKManager sharedManager] renderToFile:outputFile forDuration:duration];
In addition to opening the file with AKFileInput, you have to play it back, and to do so, you'll be creating an AKInstrument. In that AKInstrument you can process the output with any of AudioKit's signal modifier operations.