This KB article applies to you if you cannot play Windows Media Audio 9 Professional 5.1 Channel in your Stereo Speakers using the Stereo Speaker configuration in Sound and Audio Devices. Most probably because you currently have Morgan Stream Switcher and/or MatrixMixer installed in your system. It is strongly recommended that you follow the KB article on DirectShow filter problems. If you still need Morgan Stream Switcher and/or MatrixMixer installed in your system then you will have to proceed as follows:
If you experience slow play back and no sound, most probably you do not have at least 4.1 surround sound or greater.
Close Windows Media Player 9
Go to Control Panel > Sound and Audio Devices
Speaker Settings > Advanced
Change speakers to 5.1 Surround Sound Speakers
Re-play the movie