Normalize a bunch of audio files to the same loudness

I had a bunch of audio files in a directory, each recorded live with different devices, and it proved very ear-painful to hear the audio files in a playlist because of the difference of loudness.
To normalize audio filesm  you can find a number of tool working with ID3 tags, but after testing with vlc, mplayer, and the pogo mp3 player none of them did produce a measurable change. So I converted everything to wav, normalized the wav files, then converted back to mp3.

delete funny chars and spaces in file names
detox music_dir
converting files to wav is just a matter of
# this uses zsh recursive globbing
for file in **/*.mp3 ; do ffmpeg -i $file  "$(basename $file .mp3).wav"; done
normalizing files with the normalize-audio program, from the debian package of the same name.
# this uses zsh recursive globbing
normalize-audio **/*.wav
converting back to mp3
for file in **/*.wav ; do ffmpeg -b:a 192k -acodec libmp3lame -i $file "$(basename $file .wav).mp3"; done