GNU/Shoes is a wrapper for the Sox audio utility. Written in Perl for Linux and Win32, GNU/Shoes aids the tedious process of converting large numbers of audio files, and provides Jabber messages to notify the user that a batch is complete.
The Open Conversion Toolkit converts files from one type to another
OCT ("Open Conversion Toolkit") is a set of programs for converting arbitrarily many files from their own file formats to others.
It includes a simple command-line program for converting, as well as a convenient wxWidgets user interface.