Here is the API that one can use to set the default playback application of Windows Vista and Windows 7: IApplicationAssociationRegistration::SetAppAsDefaultAll Method http://msdn.microsoft.com/en-us/library/bb776338(v=VS.85).aspx This method takes only one parameter – a pointer to a null-terminated Unicode string that specifies the registered name of the application. In this case it will be “Windows Media Player”.
This is because we do not have an inbox MF A-law decoder. This is not the case for U-law. WMP12 falls back to DirectShow to playback A-law content.