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