Fixed annoying sound when pausing/shutting down (please test for ALL backends) (couldn't do this for CoreAudio and PulseAudio too)

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@4676 8ced0084-cf51-0410-be5f-012b33b47a6e

Source/Core/AudioCommon/Src/AOSoundStream.h

@@ -64,6 +64,8 @@ public:
 	
 	virtual void Update();
 
+	virtual void Mute(bool bMute);
+
 #else
 public:
 	AOSound(CMixer *mixer) : SoundStream(mixer) {}