I just had the same problem wanted to take all my music and refill my empeg , but remember that I had downloaded all from iTunes match and there for my music is mostly AAC.

I used this on my server and its working great.

http://gimpel.gi.funpic.de/wiki/index.php?title=Howto:convert_aac/mp4_to_wav/mp3/ogg_on_Linux

there is a scrip there that uses FFmpeg , and it just works.

I exported all the files into Dirs = Playlistnames and then just ran the script on the root of the dirs
then il just put my mp3 back and delete the AAC and trie to say on mp3 not arc :p

Wish theyd make an upgrade empeg id buy one, Gig Eth, prefer and AAC support.