You should list programs or packages that are required to use the script. Maybe under your version number in the head of the script would suffice.
As for your main ffmpeg command:
The native aac encoder (sometimes called ffaacenc) in ffmpeg is usually not considered to be very good. Oneiric supports libvo-aacenc if the user also installs libavcodec-extra-53. Other Ubuntu versions can use libfaac if the Medibuntu version of the same package is used (libavcodec-extra-52). Unfortunately this requires additional work by the user, although it isn't much work. Instructions for that are here:
ffmpeg -i $cfg_workingDir/album_MP3WRAP.mp3 -y -vn -acodec aac -strict experimental -ab 64k -ar 44100 -threads 3 -f mp4 "$cfg_workingDir/$ffOUT" >> "$logs" 2>&1
HOWTO: Easily enable MP3, MPEG4, AAC, and other restricted encoders in FFmpeg
I do not believe ffaacenc supports -threads. -ab 64k might be a little low, but for audiobooks perhaps it's enough.