installing ffmpeg on debian is rather tricky (you will need to compile from source).

so a command that works as „substitue“ is avconv.

sudo bash;
apt-get update;
apt-cache search libavcodec
libavcodec-extra - Libav codec library (additional codecs meta-package)
libavcodec-extra-56 - Libav codec library (additional codecs)
libavcodec56 - Libav codec library
winff - graphical video and audio batch converter using ffmpeg or avconv
libavcodec57 - Library to encode decode multimedia streams - runtime files
libavcodec-dev - Library to encode decode multimedia streams - devel files
libavfilter6 - FFmpeg filter library
libavfilter-dev - Development files for libavfilter
mencoder - MPlayer's Movie Encoder
libmythavcodec57 - libavcodec package for MythTV

apt-get install libav-tools libavcodec-extra-56;

vim /scripts/;

echo "$1";
avconv -i "$1" -c:a libmp3lame -ac 2 -q:a 2 "$1.mp3";
# save and quit :wq

chmod u+x /scripts/;

to convert aac to mp3 go a

/scripts/ filename.aac

vim /scripts/

echo "$1";
ffmpeg -i "$1" -c:a libmp3lame -ac 2 -q:a 2 "$1.mp3";