Get xmms, and make sure you can play all of the file formats. You should be able to get plugins for most formats.
Then set the output method to the disk writer plugin (should be included, but if not you can download it). You set it up and specify a directory. Then create one big playlist with all of the files you want and press play. The songs should zip through really quickly, and when it's finished the uncompressed files will be there.
Make sure you don't have repeat mode selected, otherwise it'll go on for ever.
Hope this helps