How do I find the MP3 downloads associated with a CD?
You can use the RelationshipType parameter with the RelatedItems response group
to find MP3 downloads associated with a CD (or the CD associated with an MP3 download).
For example, consider the album Human by The Killers. The ASIN of the CD version
of the album is B001FWRZ46. To find MP3 downloads of the same album, do an
ItemLookup on the CD's ASIN (B001FWRZ46) with RelationshipType=AuthorityTitle:
>>> View ItemLookup for B001FWRZ46
In the RelatedItems element, you find the ASIN of the parent - B001HCK0H8.
Do basically the same ItemLookup on the parent ASIN (B001HCK0H8):
>>> View ItemLookup for B001HCK0H8
In the RelatedItems element you'll see the ASINs of CD, LP, and MP3 versions of the album. The web service lists a maximum of 10 related items per query. If the query results in more than 10 related items, add the RelatedItemPage parameter to your query and set it to "2" to return the second page of ten related items, "3" to return the third page, etc.
You can get the ASINs of the individual MP3 tracks of the MP3 album by doing an ItemLookup on the MP3 album's ASIN (B001L8LCXY) and using the RelatedItems response group with RelationshipType=Tracks:
>>> View ItemLookup for B001L8LCXY