I added the MCFG_FLOPPY_DRIVE_SOUND(true) to src/bus/a2bus/a2diskiing.cpp and now I'm hearing drive sounds (at least the motor running anyway) if I turn the slider volume all the way to 2.0 (Can I turn it up to 11?)
There's something cool about hearing a motor sound.
I still intend (if I find enough time) to design an Arduino-based device that creates the various floppy sounds for any attached device. Eventually we should have something like a sound library for different floppy devices which can be chosen by the user. Or alternatively, a comprehensive set of floppy drive types from which to select, with the proper sound lib.