Thank you very much for your reply. However, I looked at Stamp and I don't think it does what I'm looking for. To add metadata It appears it can only edit an existing tag (aac formats have no metadata to edit), or re-compress the audio to a file format such as MP3 or Ogg which are compatible with metadata tags, but that would mean compressing an already compressed file, reducing its audio quality and/or growing the file size substantially. I'm fairly confident there is software available that just adds tags in a suitable container without processing the audio again, such as the aac audio in an m4a container, but I haven't been able to find such software, unless SSC can do it. That would indeed grow the file slightly as it would be adding the metadata, but that should be insignificant in comparison with the size of the existing aac files. But possibly licensing issues mean such software is not available to consumers?