Smart Speed like Overcast. Smart Speed means shorten the silence of audio for saving time.

SmartSpeed.swift

do {
    try audioPlayer = AVAudioPlayer(contentsOf: url)
    audioPlayer.isMeteringEnabled = true
    audioPlayer.enableRate = true
    audioPlayer.prepareToPlay()
} catch let error as NSError {
    print("audioPlayer error \(error.localizedDescription)")
}

audioPlayer.play()

//
timer = DispatchSource.makeTimerSource(queue: DispatchQueue(label: "com.domain.app.timer"))
timer.scheduleRepeating(deadline: .now(), interval: .milliseconds(100))

timer.setEventHandler { [unowned self] in

    self.audioPlayer.updateMeters()
    let peakPower = self.audioPlayer.peakPower(forChannel: 0)

    var newRate: Float = 1.0

    //increase rate if peak power is low
    let silentPower: Float = -9
    if peakPower < silentPower {
        let k: Float = 0.06
        newRate = 1 + min((silentPower - peakPower)*k, 1) //speed up (1 ... 2]
    }

    self.audioPlayer.rate = newRate
}
timer.resume()