Hmmm, is there a reason you can't just keep them up on a regular domain somewhere "in the back", or is it the sheer size and amount of files you're worried about?
S3 can get *very* expensive if there's much bandwidth and for any of the alternatives I'm aware of (for example
http://www.cachefly.com/plans.html) you're going to get dinged heavily for storage (especially if your files are averaging 1gb)
I'd suggest that the cheapest/fastest way would be to set something up on her own domain somewhere, break up the really big audios into chapters, and let people download them there. Unless she's anticipating huge amounts of traffic and high download volumes consistently I don't know that S3 is a great choice.