Amazon, perhaps the topmost global player in the book-trade, fell victim to a deplorable misunderstanding. They did not know the ISMN despite the fact that they are now dealing with notated music on a small but considerable scale. For this reason they put pressure on music publishers in a number of countries, e.g. Scandinavia and Hungary, to drop the ISMN and adopt the  ISBN - a clear breach of international standards. ISO 2108 (ISBN) clearly states that music is excluded ...
It does happen all the time that staff make mistakes, and we could just quote: O Lord, forgive them as they do not know what they are doing! and continue our work. But in the case of Amazon the situation is more serious - because of its strong position on the market the company cannot be easily reached - there are no email addresses that can be found easily, and using them would not lead to a reply; the same holds good with regular mail, not only with Seattle but also with national branches.
So does that mean that big companies can cancel international standards?
Fortunately, GS 1 the worldwide barcoding company rescued ISMN - in their update of the authoritative GTIN Validation Guide they included a clear statement that ISMN is - technically – on the same level as ISBN and is a valid identifier and transaction number. Thank you, GS 1 colleagues!
No comments from Amazon so far ... 
If there are any further difficulties please report to the International Agency.
The GTIN Validation Guide can be downloaded here.