1、

AVURLAsset* audioAsset = [AVURLAsset URLAssetWithURL:audioFileURL options:nil];
CMTime audioDuration = audioAsset.duration;
float audioDurationSeconds = CMTimeGetSeconds(audioDuration);

2、

let audioAsset = AVURLAsset.init(url: yourURL, options: nil)

audioAsset.loadValuesAsynchronously(forKeys: ["duration"]) {
    var error: NSError? = nil
    let status = audioAsset.statusOfValue(forKey: "duration", error: &error)
    switch status {
    case .loaded: // Sucessfully loaded. Continue processing.
        let duration = audioAsset.duration
        let durationInSeconds = CMTimeGetSeconds(duration)
        print(Int(durationInSeconds))
        break              
    case .failed: break // Handle error
    case .cancelled: break // Terminate processing
    default: break // Handle all other cases
    }
}

3、

NSURL * pathToMp3File = ...
NSError *error = nil;
AVAudioPlayer* avAudioPlayer = [[AVAudioPlayer alloc]initWithContentsOfURL:pathToMp3File error:&error];
double duration = avAudioPlayer.duration; 
avAudioPlayer = nil;