You can customize numerous aspects of SwiftKey Keyboard, but keypress sounds have not thus far been not one of them. As of now, they are. The latest update to SwiftKey includes the option to set one of four keypress sound profiles. There’s Traditional, Android, Modern, and Blip.
The keypress sounds are available in the keyboard settings under “Typing.” Modern sounds like a woodblock, Android replicates the default Android keypress sound, Traditional sounds like a clicky mechanical switch, and blip is a little electronic beep.
SwiftKey adds keypress sounds in latest update was written by the awesome team at Android Police.