Clean up sound devices on game exit.

OpenRA.Game/Sound/Sound.cs

@@ -20,7 +20,7 @@ using OpenRA.Traits;
 
 namespace OpenRA
 {
-	public class Sound
+	public sealed class Sound : IDisposable
 	{
 		readonly ISoundEngine soundEngine;
 		Cache<string, ISoundSource> sounds;
@@ -368,5 +368,10 @@ namespace OpenRA
 
 			return PlayPredefined(rules, player, null, type.ToLowerInvariant(), notification, variant, true, WPos.Zero, 1f, false);
 		}
+
+		public void Dispose()
+		{
+			soundEngine.Dispose();
+		}
 	}
 }