Audio Load Mp3 from PersistentData result in a shorter clip

Hello everybody, I'm facing a weird problem.

I put a snippet of my code (the file is correctly saved, in Audacity the length is right)

Code (CSharp):

1. // Assigns the loaded audio clip to the source or does nothing if the argument filename or path is inexistent.
2.  
3.     public IEnumerator<WWW> Load(string filename )
4.     {
5.         string path = Path.Combine(Application.persistentDataPath, filename + ".mp3");
6.         Debug.Log(path);
7.         if (File.Exists(path))
8.             {
9.             AudioClip clip = new AudioClip();
10.                 WWW www = new WWW("file://" + path);
11.             yield return www;
12.             Debug.Log("Sto caricando il file...");
13.             clip = www.GetAudioClip();
14.                 clip.name = filename;
15.             Debug.Log(clip.length);
16.             if (onLoadedAudioRecordedDelegate!=null){
17.                 onLoadedAudioRecordedDelegate(clip);
18.             }
19.  
20.         }
21.     }

The clip is returned fine, but the length is not right.
For example the original file length is about 3.30 seconds, the one that Unity return is about 2.74 seconds. I used UnityWebRequest too but the result is the same.

There is some way to resolve this bizzare error? Thank you!