Clearly the signal processing/audio side of the app can't be written in Lisp, and that actually makes up a pretty large chunk of the code. And having to bridge

Clearly the signal processing/audio side of the app can't be written in Lisp, and that actually makes up a pretty large chunk of the code. And having to bridge everything over FFI really isn't very practical for a UI of this complexity either for performance and code overhead reasons. Being able to access your internal data structures directly without jumping through FFI/translation hoops is a big win for both efficiency and code maintenance. So Lisp zealots can ding my comment all they want but it doesn't change the fact that intelligent, informed programmers all over the world continue to reject Lisp for very practical reasons that the true believers of the Lisp community will never understand.