One could readily create "music source" for this, however, the main issue is that Project Gutenberg does not provide a web service for the catalog (mainly for bandwidth reasons - this is a non-profit group).
However, they *do* provide the catalog in XML, which could easily be used to create a third-party webservice that serves up the catalog in bite-sized chunks - all it takes is someone to put it together and host it. I'd guess a couple of days of work for the service.
They're also not set up very well for streaming audio - their intent is that one download the audio files and store them somewhere to play them - unfortunately, a stock chumby does not have the built-in storage to hold the audio.
I'd love to see someone knock out a Project Gutenberg "music" source - however, it really needs to be done is a way that is polite to the upstream service, since they're very light on resources and heavy loads could put them out of business.