LoadLevel at the end of a sound ! (pb with Coroutine)

Hello

I need to load a level at the end of a sound. It works fine the first time but when I reload the level with my coroutine it doesn't work anymore .

I got a menu with some buttons, one button leads to the Credits of my game . When I click on the Credits button my Credits scene is working. At the end of my Credits I get back to my first menu and if I click again on my Credits Button it doesn't work . I don't understand why. Please help me.

 void OnGui()
     {
       if (GUI.Button(new Rect [...]))
        {
          _soundManager.playSfx(SoundToPlayOnClick); // Play the sound
          if (_soundManager.IsSfxPlaying() == true) // check if the sound is playing
           {
             StartCoroutine(SetActionAfterSound(LevelToCall); 
           }
        }
     }
     
     public IEnumerator SetActionAfterSound(string level)
     {
        yield return new WaitForSeconds(_soundManager.getLengthOfCurrentSfx()); 
        UnityEngine.Application.LoadLevel(level);
     }