Curlew is an easy to use, Free and Open-Source Multimedia converter for Linux.
MystiQ
Easy to use and elegant Multimedia converter.
Convertitore di sonu
soundKonverter is a frontend to various audio converters.
DeaDBeeF
DeaDBeeF lets you play variety of audio formats, convert between them, customize the UI almost any way you want, and use many additional plugins which can extend it even more.
Cianu
The ciano is a multimedia converter for all the format you need.