Is it possible to read out the “Album Artist” ID3 tag with spotlight query

At the risk of stating the obvious, 'sort'.

e.g. mdfind -onlyin "$MUSICROOT" -literal "$MDQUERY" | sort

will sort the output alphabetically, giving you Artist, Title, Track. (it won't do the cool iTunes trick of ignoring 'The').

The sort command has lots of options, including sorting on particular fields if you want more control.

Edit: The below shell script function will provide the desired sort in the specific case mentioned:

function sort_file()
{
    TMPFILE=`mktemp /tmp/sortable.XXXXXX` || exit 1
    export MDLS="mdls -raw -name kMDItemAlbum -name kMDItemAudioTrackNumber"
    export TR="tr \"\0\" \"\;\""
    awk '{system( "$MDLS \"" $0 "\" | $TR" ); print "\;"$0; }' $1 > $TMPFILE
    sort -t\; -k 2n -k 1f $TMPFILE | awk -F";" '{ print $3; }' > $1
}

I prefer the mutagen Python library for tag editing, since it has good support for a wide variety of formats with a consistent interface. Here’s a script for copying the metadata of a file:

import mutagen
import mutagen.easyid3
old = mutagen.easyid3.EasyID3('example.mp3') 
new = mutagen.File('example.m4a') 
new.update(old)
new.save()

You can open any file with mutagen.File, but that gives you raw access to the coded ID3 tags like 'TPE1', instead of human-readable tags like 'artist', so EasyID3 is used instead.