AVAudioUnitTimePitchでうまくいくはずです。
udacity.com Intro to iOS App Development with Swiftのサンプル コード:
func playAudioWithVariablePitch(pitch: Float) {
audioEngine.stop()
audioEngine.reset()
let audioPlayerNode = AVAudioPlayerNode()
audioEngine.attachNode(audioPlayerNode)
let changePitchEffect = AVAudioUnitTimePitch()
changePitchEffect.pitch = pitch
audioEngine.attachNode(changePitchEffect)
audioEngine.connect(audioPlayerNode, to:changePitchEffect, format:nil)
audioEngine.connect(changePitchEffect, to:audioEngine.outputNode, format:nil)
audioPlayerNode.scheduleFile(audioFile, atTime:nil, completionHandler: nil)
try! audioEngine.start()
audioPlayerNode.play()
}