for i in *.m4a; do ffmpeg -i "$i" -c:a aac -b:a 256k "${i%.*}.aac"; done
To determine if alac or acc:
for i in *.m4a; do t=$(ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 "$i"); echo $t $i ; done
In a mixed directory, to only convert alac to aac:
for i in *.m4a; do t=$(ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 "$i"); if [ $t = 'alac' ]; then ffmpeg -i "$i" -c:a aac -b:a 256k "${i%.*}.aac" ; fi; done;