I appreciate that you want to convert/amplify an aac. but have you considered using an MP3? I've used mp3gain for years with good results. The best bit is it's totally FREE.
you can make your mp3s louder with no loss of quality.
(you can apparently use the software to do the same with an AAC. file but i've never used it for that and don't know if it works as well as the mp3 bit)
It's an old program now but it's a goodie!
Google is your friend!
(or use this link
MP3Gain )
Hope this is helpfull