Hello Stephen,
your project sounds very interesting, I hope to see (hear) some results soon!
The Wikimedia Commons is indeed the correct place, and your "Con" for it is not a Con at all:
Con: Unsure if it's appropriate for this. Commons is intended for shared media resources across Wikimedia projects, and I can't see how (for example) the Catalan Wiktionary would need an audio version of Chapter 7 of Wuthering Heights.
A file on the Commons currently needs to be *potentially* useful in at least *one* Wikimedia context. Your files could be linked from Wikisource, but also from Wikibooks and Wikipedia.
Generally, if it's a file, and it's free content, it belongs in the Wikimedia Commons so it can be easily used everywhere.
However, your case underscores my standing argument that Wikisource and Wikimedia Commons should be merged, since they both serve very similar purposes (repositories of free content), and transformations like yours would be natural in a single repository.
Regards,
Erik