The Classic II is capable enough to control a rig full of midi eqipment. Professional musicians did use it for this purpose. Appropriate software should still be available somewhere, as well as interface modules are available and supported by recent digital instruments. Using the Classic II as a synthesizer would create a retro style sound experience, but using it as a controller for external synthsizers still should be fine.

The Mathematica software allows to produce sound output from calculations. Just define a periodical function and listen to it. Version 2 should run well on a Classic II.