iTunes/Music/${author}/${album}/${song.mp3}

#!/bin/bash
SAVEIFS=$IFS
IFS=$(echo -en "nb")
for f in *.mp3
do
  lame --cbr $f __out.mp3
  mv __out.mp3 $f
done
IFS=$SAVEIFS

$ cd iTunes/Music
$ global normalize_mp3.sh

global() {
  shopt -s globstar
  origdir="$PWD"
  for i in **/; do
    cd "$i"
    echo -n "${PWD}: "
    eval "$@"
    echo
    cd "$origdir"
  done
}

global normalize_mp3.sh

$ echo $BASH_VERSION
4.1.5(1)-release
$ cd /var
$ tree -d | head
.
├── backups
├── cache
│   ├── apt
│   │   ├── apt-file
│   │   └── archives
│   │       └── partial
│   ├── cups
│   │   └── rss
│   ├── debconf
$ global 'echo -n This command is executed in $PWD at $(date); sleep 1'
/var/backups: This command is executed in /var/backups at Mo 1. Jul 12:11:00 CEST 2013
/var/cache: This command is executed in /var/cache at Mo 1. Jul 12:11:01 CEST 2013
/var/cache/apt: This command is executed in /var/cache/apt at Mo 1. Jul 12:11:02 CEST 2013
/var/cache/apt/apt-file: This command is executed in /var/cache/apt/apt-file at Mo 1. Jul 12:11:03 CEST 2013
/var/cache/apt/archives: This command is executed in /var/cache/apt/archives at Mo 1. Jul 12:11:04 CEST 2013
/var/cache/apt/archives/partial: This command is executed in /var/cache/apt/archives/partial at Mo 1. Jul 12:11:05 CEST 2013
/var/cache/cups: This command is executed in /var/cache/cups at Mo 1. Jul 12:11:07 CEST 2013
/var/cache/cups/rss: This command is executed in /var/cache/cups/rss at Mo 1. Jul 12:11:08 CEST 2013
/var/cache/debconf: This command is executed in /var/cache/debconf at Mo 1. Jul 12:11:09 CEST 2013
...

find ~/Music -name *.mp3 | while IFS= read -r f; do
  lame --cbr "$f" "$f"_temp
  mv "$f"_temp "$f"
done

mdfind 'kMDItemAudioBitRate>128000' -onlyin ~/Music |
parallel lame --cbr {} {}_temp ; mv {}_temp {}

f() { lame --cbr "$1" "$1"_temp; mv "$1"_temp "$1"; }
export -f f
mdfind 'kMDItemAudioBitRate>128000' -onlyin ~/Music | parallel f

shopt -s globstar # bash 4.0 or later
for f in ~/Music/**/*.mp3; do lame --cbr "$f" "$f"_temp; mv "$f"_temp "$f"; done

#!/bin/bash
# first cd to iTunes/Music
for f in */*/*.mp3
do
  lame --cbr "$f" __out.mp3 && mv __out.mp3 "$f"
done

#! /usr/bin/env bash
for f do
  lame --cbr "$f" "$f_out.mp3"
  mv "$f_out.mp3" "$f"
done

downrate_mp3 ./*.mp3

shopt -s globstar
downrate_mp3 ./**/*.mp3

find iTunes/Music -type f -name '*.mp3' -exec downrate_mp3 {} +