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/DSoundStream.h

@@ -79,6 +79,7 @@ public:
 	virtual void SetVolume(int volume);
     virtual void Stop();
 	virtual void Clear();
+	virtual void Mute(bool bMute);
     static bool isValid() { return true; }
     virtual bool usesMixer() const { return true; }
     virtual void Update();