This neat trick, as discovered by the folks at Yamaha, can be achieved without any overly complicated mods or hacks, as it works over Apple’s wireless AirPlay technology. The Siri-powered music is made possible by sending a converted song file over Wi-Fi to an Apple Airport Express base station, which then passes the information on to the attached Yamaha Disklavier piano.
The only tinkering needed to get a Siri-controlled musical performance like this is to make use of special song files. These special song files have MIDI data maintained within them, meaning the Yamaha piano can make sense of any files sent to it. Play it again, Siri.