Audio Interchange File Format was created on 1988-01-21.
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wav is a file format or more precicely a container for audio files, it is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs. It is an application of the RIFF bitstream format method for storing data in "chunks".
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Mp3 is a file format commonly used for audio files. This is not
the only format but is a very common one. An MP3 file, an audio
file, is already a song. There is no possible way of converting an
MP3 song to an audio song because 'audio' is not a format to
convert to.
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To change your Windows Movie Maker audio file name extension (format) to .wma all you need to do is Publish/Finishit.
Published video is exported in .wmv format, while audio is mixed-down into .wma format.