It is important to distinguish between a file format and a codec. A codec performs the encoding and decoding of the raw audio data while the data itself is stored in a file with a specific audio file format. Though most audio file formats support only one audio codec, a file format may support multiple codecs, as AVI does.
XRECODE is a great multilanguage free software for audio encoding. Look at this clear picture to understand:
It can convert multiple files in parallel using multi-core CPU.
It is very simple.
It is very efficient.
Full unicode support for input and output files under Win2K,XP,VISTA.
Confirmed to run under linux via Wine (Ubuntu 8.04)