fix audio thread race condition on StaticAudio.GetInstance

src/Audio/StaticSound.cs

@@ -271,24 +271,43 @@ namespace MoonWorks.Audio
 		/// NOTE: If AutoFree is false, you will have to call StaticSoundInstance.Free() yourself or leak the instance!
 		/// </summary>
 		public StaticSoundInstance GetInstance(bool autoFree = true)
+		{
+			StaticSoundInstance instance;
+
+			lock (AvailableInstances)
 			{
 				if (AvailableInstances.Count == 0)
 				{
 					AvailableInstances.Push(new StaticSoundInstance(Device, this));
 				}
 
-			var instance = AvailableInstances.Pop();
+				instance = AvailableInstances.Pop();
+			}
+
 			instance.AutoFree = autoFree;
+
+			lock (UsedInstances)
+			{
 				UsedInstances.Add(instance);
+			}
+
 			return instance;
 		}
 
 		internal void FreeInstance(StaticSoundInstance instance)
 		{
 			instance.Reset();
+
+			lock (UsedInstances)
+			{
 				UsedInstances.Remove(instance);
+			}
+
+			lock (AvailableInstances)
+			{
 				AvailableInstances.Push(instance);
 			}
+		}
 
 		protected override unsafe void Destroy()
 		{