Thanks to ksv and anerki (and some others): People, re-encoding your music from MP3 to AAC damages your music files on a large scale !.
Have you ever re-saved a JPEG as another JPEG file ? You're doing just the same.
Look. MP3 is a lossy format. A very lossy one, I'd say, try to encode some jazz under 256kbps and you'll understand.
Now, AAC is lossy too. Less, but still, it is.
Now, figure that out: what happens when you re-encode an already damaged file ?
You're just multiplying the damage ! (MP3 damage) x (AAC damage) !
MP3 compression is already eating your sound, if you have re-encoded files, I regret, but you have killed your whole library. The sound will be sh!tty unless you stick to those poor speakers iMacs and portables come with.
Just try this: leave a message on a friend's mobile phone and ask him to call you and to make you listen to the message by the phone (see whatta mean ? :confused) That's double sound damaging, same thing.
So, last: do NOT re-encode your music ! Unless your sole musical interest is Madonna and Britney Spears, any other music will get killed in the process. Barbara Hendricks will sound like Gerry Halliwell.