Using faad, convert the m4a to wav:
$ faad -o abc.wav abc.m4a
Yes you read that right - faad has output as the first option, and input as the second option.
Then using lame convert wav to mp3. Bitrate is specified with the "-b" option.
$ lame -h -b 192 abc.wav abc.mp3
You can combine this into a script that will convert all m4a's in a directory into mp3s
for i in *.m4a
do
faad -o - "$i" | lame -h -b 192 - "${i%m4a}mp3"
done
