Encoding Multichannel Audio
You can encode multichannel audio for a surround-sound playback in six (5.1 audio) or eight (7.1 audio) channels. The format is specifically designed for CD, DVD, high definition television, and digital cinema audio applications. Your audio source must be one of the following:  
 
A single 6-channel or 8-channel WAVE_FORMAT_EXTENSIBLE file. If you are sourcing from an 8-channel file, it is recommended that the sampling rate and bits per sample of the source file and the encoded content match.  
Six mono channel WAV files. First, use an audio editing program to fold down your audio to six files, one for each channel. Then, when you set up your source in the encoding session, specify which channel each file is associated with. Each WAV file cannot exceed 2 gigabytes (GB) in size.  
One way to set this up is by using Windows Media Encoder; specify the mono channel WAV files and then save the configuration to a .wme file. Then, load the configuration using the Load method from IWMEncoder or WMEncoder. You can also set the mono channel WAV file using the property bag (IPropertyBag). The properties are listed in the following C++ example.  
 
An existing multichannel Windows Media Audio (WMA) file to encode at a different bit rate. This feature does not work on a computer running Windows 2000.  
You must use the Windows Media Audio 9 Professional codec and an audio format that supports multichannel encoding when encoding multichannel content. In addition, the user playing your content must have Microsoft Windows XP and a player that is based on the Windows Media Format 9 Series Software Development Kit (SDK). Otherwise, the audio will be folded down automatically to two channels for stereo speakers.
 
The encoding process automatically folds the channels down to two for playback with stereo speakers. If you are encoding 5.1 audio, you can control the fold-down distribution between the surround, center, and subwoofer channels. Add an attribute for each channel using IWMEncAttributes; use the attribute names in the following table, and specify attribute values indicating the decibels (between 0 to -144).
 
Channel Attribute name  
Surround mix WMENC_AUDIO_SURROUND_MIX  
Center mix WMENC_AUDIO_CENTER_MIX  
Low-frequency effects mix WMENC_AUDIO_SUBWOOFER_MIX