Recently got a Nintendo DSi for my child and one of the first things she asked was for her music to be copied there.
Since the device does not read mp3 files, we had to get them converted to aac, despite the fact that they have to have the m4a extension.
ffmpeg does a great job for converting several file formats, so after some googling I got this one line to convert all the stuff in one folder:
for f in *.mp3; do echo $f ; ffmpeg -i "$f" -acodec aac -ab 256k "${f%mp3}m4a" 2>/dev/null ; done
And it works like a charm 🙂