Swift + AVAudioRecorder records very quietly

I have built an iOS App in Swift that records audio clips, these clips are then sent up to the server. Every recording I make is very quiet.

Initially I thought my problems were similar to this question on Stack Overflow - but after trying this solution, my recordings are still very quiet.

Routing the audio through the speaker does not make the recordings any louder, as is suggested here:

http://ift.tt/1ANvrwS

Note that its not playback on the device that is an issue. The issue is that the recordings are too quiet. I have tried with a variety of microphones in both a Mac and a PC. All the microphones are much louder (sensitive to sound?) on the PC than the Mac.

This is the best waveform I can produce, and this is almost shouting into the Microphone. As you can see, the waveform recorded is very quiet:

Is there any way of getting my iOS App to record at a louder volume?

This is the code I am using to record the audio clips:

let session: AVAudioSession = AVAudioSession.sharedInstance()
var error: NSError?
if session.respondsToSelector("requestRecordPermission:") {
    AVAudioSession.sharedInstance().requestRecordPermission( { (granted:Bool) -> Void in
        if !granted {
            println("permission not granted")
        }else{
            println("permission granted")

            if !session.setCategory(AVAudioSessionCategoryRecord, error: &error) { // also try PlaybackAndRecord
                println("could not set sesssion category")
                if let e = error {
                    println(e.localizedDescription)
                }
            }

            if !session.setActive(true, error: &error) {
                println("could not make session active")
                if let e = error {
                    println(e.localizedDescription)
                }
            }

            if !session.overrideOutputAudioPort(AVAudioSessionPortOverride.Speaker, error: &error) {
                println("could not override output audio")
                if let e = error {
                    println(e.localizedDescription)
                }
            }

            if !session.setActive(true, error: &error) {
                println("could not make active")
                if let e = error {
                    println(e.localizedDescription)
                }
            }

            self.currentFilename = "xxxx.wav"
            let dirPaths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
            let docsDir: AnyObject=dirPaths[0]
            self.recordedFilePath = docsDir.stringByAppendingPathComponent(self.currentFilename)
            self.recordedFileURL = NSURL(fileURLWithPath: self.recordedFilePath)

            self.recorder = AVAudioRecorder(URL:self.recordedFileURL, settings: self.recorderSettings, error: &error)

            if let e = error {
                println(e)
                println(e.localizedDescription)
                var err = NSError(domain: NSOSStatusErrorDomain, code: e.code, userInfo: nil)
                println(err.description)
            }else{

                self.recorder?.delegate = self
                self.recorder?.meteringEnabled = true
                self.recorder?.prepareToRecord()

                self.recorder?.record()

                self.meterTimer = NSTimer.scheduledTimerWithTimeInterval(0.1,
                    target: self,
                    selector: "updateRecordAudioMeter:",
                    userInfo: nil,
                    repeats: true)

            }
        }
    })
}