Here is an alternative code sample that will encode the file as AAC inside an m4a:

NSArray *dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docsDir = [dirPaths objectAtIndex:0];
NSURL *tmpFileUrl = [NSURL fileURLWithPath:[docsDir stringByAppendingPathComponent:@"tmp.m4a"]];
NSDictionary *recordSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                        [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                        [NSNumber numberWithFloat:16000.0], AVSampleRateKey,
                        [NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
                        nil];
NSError *error = nil;
AVAudioRecorder *recorder = [[AVAudioRecorder alloc] initWithURL:tmpFileUrl settings:recordSettings error:&error];
[recorder prepareToRecord];

AVAudioSession *session = [AVAudioSession sharedInstance];
[session setCategory:AVAudioSessionCategoryRecord error:nil];
[session setActive:YES error:nil];

[recorder record];

Then to end the recording I used:

[recorder stop];
AVAudioSession *session = [AVAudioSession sharedInstance];
int flags = AVAudioSessionSetActiveFlags_NotifyOthersOnDeactivation;
[session setActive:NO withFlags:flags error:nil];

Then the file at tmpFileUrl can be used.

Here is the working SWIFT code to record m4a Audio files. Bear in mind that hitting the right format parameters that produce usable audio files in iOS is really painful to find. I found that this combination works, after much trial and error. I hope it saves you time, enjoy!

let recordSettings: [String : AnyObject] = [AVSampleRateKey : NSNumber(float: Float(16000)),
                                                AVFormatIDKey : NSNumber(int: Int32(kAudioFormatMPEG4AAC)), 
        AVNumberOfChannelsKey : NSNumber(int: 1),
        AVEncoderAudioQualityKey : NSNumber(int: Int32(AVAudioQuality.Low.rawValue))]

func initializeAudioSession(){


    let audioSession = AVAudioSession.sharedInstance()
    do {
        try audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord)
        try audioRecorder = AVAudioRecorder(URL: self.directoryURL()!,
                                            settings: recordSettings)
        audioRecorder.delegate = self
        audioRecorder.meteringEnabled = true
        audioRecorder.prepareToRecord()
    } catch let error as NSError{
        print("ERROR Initializing the AudioRecorder - "+error.description)
    }
}

func recordSpeechM4A(){
        if !audioRecorder.recording {
            let audioSession = AVAudioSession.sharedInstance()
            do {
                try audioSession.setActive(true)
                audioRecorder.record()
                print("RECORDING")
            } catch {
            }
        }
    }

func directoryURL() -> NSURL { //filename helper method
        let fileManager = NSFileManager.defaultManager()
        let urls = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
        filepath = urls[0]
        let documentDirectory = urls[0] as NSURL
        print("STORAGE DIR: "+documentDirectory.description)
        //print("---filepath: "+(filepath?.description)!)
        let soundURL = documentDirectory.URLByAppendingPathComponent("recordedAudio.m4a") //.m4a
        print("SAVING FILE: "+soundURL.description)
        return soundURL
    }