如何使用AVAudioRecorder录音 如下：
// 录音存储的url
NSURL *fileUrl = [NSURL fileURLWithPath:@"/Users/midas/Desktop/sounds.m4a"];
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
// 采样率
[dict setObject:@44100.0f forKey:AVSampleRateKey];
// 编码格式
[dict setObject:@(kAudioFormatAppleLossless) forKey:AVFormatIDKey];
// 声道数目
[dict setObject:@1 forKey:AVNumberOfChannelsKey];
// 质量
[dict setObject:@(AVAudioQualityLow) forKey:AVEncoderAudioQualityKey];
_recorder = [[AVAudioRecorder alloc] initWithURL:fileUrl settings:dict error:nil];
// 录制5s
[_recorder recordForDuration:5];
[_recorder prepareToRecord];
// 开始录制
[_recorder record];
