MacNifty produced a sound digitizer for use on the first 512K, which plugged into the serial port. You could feed a microphone or a line input into it and through the Mac's speakers. This would of course work on the Classic as well. Surely there were other such digitizers and serial microphones and software developed for the Mac thereafter, I believe Berkeley made one too ... Whether other software, like a sound visualizer would be able to access the port is another matter. But I would think someone made a solution for the compacts. One thing to consider, since that would be a very cool idea for your setup would be to put a Mac II board inside it (or just get yourself a Mac II eventually).

Of course the main thing these old Macs are good for, in addition to a cocktail database, would be as a clock. You could also install other screensavers, which are not quite as fun in Black & White, but still more fun than nothing.

You could also keep an eye out for telephone contact software and hardware that allows you to dial your phone directly from the Mac's address book.