You can use the open source ffmpeg program to convert a media file from one format to another.
Below is an exmple:
[shellprompt]# /fullpath/to/ffmpeglibrary/ffmpeg -i InputFile.FLV OutputFile.mp3
There are various other options you can use to specify the attribute of the output file.
Here are few of the examples code http://linuxers.org/book/export/html/593