Remove all 21 of the screws from the back of the piano to open it up.

Open the back from the top, being mindful not to pull on the wires that connect the battery box to the board.

Audio Out

You'll intercept the audio output of the Meowsic PCB to run through the switch, attenuator, and output jack you'll be adding.

This red wire running to the speaker is the audio out line, you can desolder this wire from the speaker now.

Ground

You'll solder the ground wire for the added circuit to this center common pin on the Meowsic on/off switch.

Selector Switch

Solder three wires to the SPDT switch -- you'll connect the common line to the audio output of the Meowsic PCB, one outer leg back to the speaker, and the other leg to the resistor-attenuator-capacitor output circuit.

Optionally, you can use a guitar pedal-style 3PDT stomp switch here, such as this one, seen in the remaining photos in this guide.

Attenuator

It's helpful to have a way to adjust the output level when interfacing with other gear. You'll used a 100KΩ potentiometer as a signal attenuator. You can wire it up as shown, with ground on the left leg, the incoming audio signal (post switch and current limiting resistor) on the right leg, and the attenuated output audio signal on the center wiper leg.

Output Jack

You have a few options for the output jack. A typical guitar pedal/amp setup will use a 1/4" mono jack. 

You can also move down to a very similar 1/8" (a.k.a., 3.5mm) mono jack, shown here. Solder the ground wire to the sleeve contact (the shorter one). Solder the signal wire to the tip contact (the longer one).

One other option, if you plan to plug into more typical 3.5mm jacks on powered speakers, is to use a 3.5mm stereo jack with both of the tip and ring legs soldered to the signal wire, and sleeve to ground.

Circuit

You can use a small PermaProto board to make all of the connections and add the 100KΩ resistor and 10µF capacitor to the circuit as shown here and in the Fritzing diagram and schematic.

Note: the two thin black wires in the first photo are unnecessary -- I soldered in both a 1/4" jack and 3.5mm jack, hence the extra connection.

Use an existing screw from the Stop button to mount the board in the case.

Assembly

Use a drill and appropriate sized bits (or a hole reamer) to make holes in the Meowsic case for the switch, knob, and jack.

Audio Jack Mounting

Mark and drill a hole for your chosen audio jack.

Potentiometer Mounting

Mark and drill a hole for your potentiometer. You'll also need to make a small secondary hold for the tab that prevents the body of the potentiometer from rotating.

You can find this second hole's position by putting the pot in from the outside and marking the cat's paint with some pressure on the small tab.

Knob

Screw on the potentiometer nut, then add the knob of your choice to the potentiometer shaft.

Here's a look at all the mods in place.