Piano sound, and more...
I connected my MacBook Pro to Casio PX-780, connect via USB then MacBook audio headphone jack connected to PX-780 Line IN. You will need adaptors (from Mic Plug to stereo mini-plug). Note: You can also use your iPhone and iPad (You will need Lightning to Data Adaptor that have USB in and power port), it works well for me, too.
Garageband on my MacBook automatically recognize Casio USB MIDI connection. From there you can select a ton of instruments. For piano have the preselected Grand Piano or you can pick Steinway Piano ... and Electric Keyboards, Synths, Guitars ... You can also do sound shaping (Tremolo, Delay, EQ, etc)
The sound will be layered (combining instruments from both PX-780 and Garageband) as default. To isolate Garageband instruments only, you must turn off Casio PX-780 instruments. To do this, press FUNCTION then use selector < > until you get MIDI, then press ENTER, then press selector <> in MIDI set until you get to LOCAL [OFF]. This now will have the audio output of MacBook coming out of PX-780 without the PX-780 instruments. Basically you get a MIDI workstation using PX-780 as your input.
With this setup you can do a whole range of music production, live looping or just using garage band instruments.
I particularly like GarageBand Classical Acoutic Guitar where it mimics the fret sound, pitch bends (by pressing hard on the key) and more.