set theFolder to quoted form of POSIX path of (choose folder)
do shell script "find " & theFolder & " -prune -name '*.m4a' -exec mp4box -raw 1 '{}' \\;"
set theFolder to (choose folder)
tell application "Finder" to set theFiles to files of theFolder whose name extension is "aac"
repeat with oneFile in theFiles
set newName to quoted form of POSIX path of ((theFolder as Unicode text) & text 1 thru -5 of name of (info for (oneFile as alias)) & ".m4a")
do shell script "mp4box -add " & quoted form of POSIX path of (oneFile as alias) & space & newName
end repeat