Not quite sure what you want to do. MPEG-4 is standard for video files with audio. There are commercial and shareware applications available that claim
to extract the audio from, an MPEG-4 video and save the audio as an MP3 or WMA file.
AAC is the audio Codec portion of the MPEG-4 standard (
http://www.apple.com/quicktime/technologies/aac/ ) and is used by iTunes for encoding purchased music files. Purchased music file include Digital Rights Management (DRM) protection and have a
.m4p filename extension. iTunes files that do not have DRM have a
.m4a filename extension.
There is no legal way to convert m4p files directly to mp3 or wma. iTunes does allow you to legally burn an audio CD from w4p files. You can then rip the audio CD to mp3 or wma files.
There are commercial and shareware applications that will convert m4a file to mp3 and wma formats. As far as I can tell Windows Media Player does not support m4a files and I am not sure if any third party plug-in is available.
You may want to look into the Pro version of Winamp (
http://www.winamp.com/player/ ) for $20. It handles m4a files as well as mp3 and wma types.
Also, as far as I know changing the portion of a filename after the dot (the filename extension) does not change the format of the contents of the file in any version of MS-Windows. If you have a file called
MySong.m4a and change the name to
MySong.mp3, the contents of the file do not change.