How can I play a custom sound in my custom keyboard

I want to play an audio file when a key on my custom keyboard is clicked using swift. I've tried this, however, have had no luck. How can I make this work? Is playing a custom audio sound on a keyboard action possible?

func btnPressed(sender: AnyObject)
{
    var btn = sender as UIButton
    var stringToInsert=""
    var proxy = self.textDocumentProxy as UIKeyInput

    switch (btn.tag)
    {
    case 10:
        stringToInsert = "a"
        if let path = NSBundle.mainBundle().pathForResource("8bit", ofType: "wav") {
            audioPlayer = AVAudioPlayer(contentsOfURL: NSURL(fileURLWithPath: path), fileTypeHint: "wav", error: nil)

            if let sound = audioPlayer {
                sound.prepareToPlay()
                sound.play()
                println("hello") 
            }
        }
    }
    proxy.insertText(stringToInsert)  
}
}