The Voice Machine was a Mechanist recording device similar to a Wax Cylinder Machine. It was used by Karras to record his speeches and play them at a party at Angelwatch. A total of six have been used during the party (and one secret one for a special guest).[1]
It's not clear why Karras decided to use these machines but he seems to knew that Garrett would attend his party,[2] and probably did not want to give him a chance to kill him; so he might have decided to stay away from the party and use the Voice Machines as a substitute for himself to speak to his guests.