Get instance of XAudio2 created by the sound engine at initialization.
- Non-addref'd pointer to XAudio2 interface. NULL if sound engine is not initialized or XAudio2 is not used. The returned pointer can be of either XAudio 2.7, XAudio 2.8, Xaudio 2.9 depending on the Windows version the game is running on. Use QueryInterface to identify which one and cast appropriately