From LinuxSwords Wiki

convert mp3 into audiobook format

Needs mpg123 and faac

mpg123 -s input.mp3 | faac -b 80 -P -X -w -o output.m4b -

in a oneliner for all files in the current directory:

for i in $(ls *mp3);  do mpg123 -s $i | faac -b 80 -P -X -w -o ${i%mp3}m4b -; done