I needed to wrap an AAC file in an M4A container so I could load it into my Windows Phone 7 (WP7) music collection. First I downloaded the FFmpeg software from here:
Then I unzipped the files and ran the following command:
C:\FFmpeg\bin>ffmpeg -i "song.aac" -acodec copy -absf aac_adtstoasc "song.m4a"
That solved the problem of not being able to add an “MP3” tag to my audio file.