iOS 播放录音声音很小

很可能你录音时AVAudioSession设为了AVAudioSessionCategoryPlayAndRecord，录音结束时没有设回来
AudioSessionCategoryPlayAndRecord是用听筒播放，所以声音会很小

let session = AVAudioSession.sharedInstance()
do {
    try session.setActive(true)
} catch let error as NSError {
    print("无法访问您的麦克风")
    return
}
if session.category != AVAudioSessionCategoryPlayback {
    try session.setCategory(AVAudioSessionCategoryPlayback)
}