Not just to amplify but also for the theremin to work properly. USB audio interfaces to "desktop" computers such as Behringer's UMC range should work. Hi-fi, active computer speakers can work but may need an appropriate lead.
A great little Theremin with lots of control (Antenna sensitivity/Master Volume/Filter/Pitch etc)
Easy to set up, and great fun to play.
Definitely recommend this little beauty! - Great build quality
Learning 'happy birthday' on it so I can play it to my sister. She'll get the half decent version. The poor neighbours have heard everything in-between. Lol