After a while, I figure out a simple way to convert files from a format to an other. For this I used faad (an open source MPEG-4 and MPEG-2 AAC decoder) and lame the well known mp3 encoder.
Both install nicely on Mac OS.
A typical transcoding sequence is as follows:
faad -o sample.wav sample.m4a
lame -h -V 6 sample.wav sample.mp3