The Casio Pt-1 Keyboard that I used in the video is modified or circuit bent. Normally the pitch of the keyboard wouldn't be able to go that low, but ...
Step 2: Use Duct Tape to Protect your Soldering
The Soldering on your speaker, and speaker itself is pretty delicate... so it's a good idea put duct tape over the back of the speaker and wires to protect them from being damaged.
(don't put tape anywhere on the front of the speaker or it will mess up the sound).