Примерно так:
tell application "iTunes"
set theTracks to (item 1 of (get selection))
set theTracks to selection
repeat with theTrack in theTracks
set albumartist to artist of theTrack
set album artist of theTrack to albumartist -- the missing line
end repeat
end tell
В реальной жизни я бы, вероятно, пропустил промежуточную переменную и записал бы пару строк одной строкой, например:
set album artist of theTrack to (get artist of theTrack)
Или даже лучше:
tell theTrack to set album artist to (get artist)