Streaming audio bug ?

I'm trying to force Unity to play audio mp3 file while it's streaiming, but it doesn't play until the end of download.
Here is a strange part: If I press right-click and open any context menu in UNITY EDITOR, it will play audio while downloading the file.

Am I doing something wrong or it's a unity bug ?

Code (CSharp):

1. using System.Collections;
2. using UnityEngine;
3.  
4. [RequireComponent(typeof(AudioSource))]
5. public class AudioPlayer : MonoBehaviour
6. {
7.     string urlMp3 = "";
8.  
9.     public AudioSource source;
10.  
11.     private void Start()
12.     {
13.         source = GetComponent<AudioSource>();
14.         InitButton(urlMp3);
15.     }
16.  
17.     public void InitButton(string url)
18.     {
19.         urlMp3 = url;
20.         progress = 0;
21.         source.time = 0f;
22.         StartCoroutine(StartDownload());
23.     }
24.     public float progress = 0;
25.  
26.     IEnumerator StartDownload()
27.     {
28.         WWW www = new WWW(urlMp3);
29.         print("start");
30.         while (!www.isDone)
31.         {
32.             progress = www.progress;
33.  
34.             source.clip = www.GetAudioClip(false, true);
35.             //source.clip.name = "Abo";
36.             if(!source.isPlaying) source.Play();
37.             print("try to play " + source.isVirtual); // must not be virtual
38.             yield return null;
39.         }
40.  
41.         if (null != www.GetAudioClip(false, true, AudioType.MPEG))
42.         {
43.             source.clip = www.GetAudioClip(false, true);
44.             print("play after download");
45.  
46.             source.Play();
47.         }
48.      
49.         source.time = 0f;
50.      
51.      
52.     }
53.  
54. }
55.