It would be Apple's (or the bankruptcy court's) responsibility to transfer the access to a competitor's service, in exchange for payment or liquidation proceeds.

In fact, I'd argue that this ought to be the norm -- that content providers be forced to purchase insurance or something precisely in case they go out of business, to ensure customers will have their content migrated to another service, and be refunded for the small proportion of content that isn't available anywhere else.

The same way it works when a bankrupt company can't pay an invoice. You can sue a company if it doesn't hold its financial obligations, and get your money, and so can the customer, like in this case.

When a company goes bust, if you are owed money by that company, you put in claims to the estate, and the administrators handle those claims according to a well specified priority list. The customers being reimbursed is part of that process. However, I know that gift cards and store credit tend to be at the bottom of the list, so I wouldn't have high hopes for digital assets being reimbursed either.