avoid race condition on StreamingSound.Dispose

src/Audio/StreamingVoice.cs

@@ -149,6 +149,8 @@ namespace MoonWorks.Audio
 		protected override unsafe void Dispose(bool disposing)
 		{
 			if (!IsDisposed)
+			{
+				lock (StateLock)
 				{
 					Stop();
 
@@ -160,6 +162,7 @@ namespace MoonWorks.Audio
 						}
 					}
 				}
+			}
 			base.Dispose(disposing);
 		}
 	}