The C code that encodes the audio is listed in the dependencies section [1], and ADTPro is freely acknowledged there. However, I'm more than happy to add some reference to the post because I think you're right.
EDIT: Aaaaaand acknowledgement pushed into post. Cheers.
Thanks very much for the acknowledgement. Your project looks like a lot of fun - very cool use of audio. And of course Lisp rocks. Another audio-based project is the Apple Disk Server - loading games directly to the Apple via cassette port:
http://asciiexpress.net/diskserver/readme.html
EDIT: Aaaaaand acknowledgement pushed into post. Cheers.
[1]: Specifically located here: https://github.com/hausdorff/apple2e-audio-transport