Thev2.8W class-D ST2012 amp will be mounted to the lower right side of the bottom enclosure part. Place the amplifier over the bottom enclosure part and line up the mounting holes with the stand-offs.
Hold the amp down to the enclosure and fasten two #4-40 3/8 Phillips flat machine screws to mount the component.
Measure two pieces of wire for connecting the ST2012 amp to the PowerBoost 500C. Pins VDD and GND on amp will connect to the +positive and -negative pins on the PowerBoost 500C. Cut two pieces of 30AWG wire. Strip and tin the ends.
Solder wires to VDD and GND pins on ST2012. Connect wires to +positive and -negative pins on PowerBoost 500C and solder.
Strip the red and white wires from the audio cable and tin the tips. Insert the white wire into the L+ pin on the ST2012 amp. Insert the red wire to the R+ pin. The ground wire needs to be connected to the L- or R- pin.
Create a small wire to jump the L- and R- pins on the ST2012 amp. Ensure it's a short piece of 30AWG wire.
The ground wire may be too thick to fit in the through-hole on the amp PCB. Carefully thin the wire by trimming the edges with wire cutters. Insert the ground wire to either L- or R-, sharing the wire jumper.
Measure the lengths of wires needed for connecting the speakers to the amplifier. Place the top enclosure part near the bottom part and lay speakers over the cavities to guage wire length - The left and right speakers will have different lengths!
Measure again, and cut four pieces of 30AWG wire making two sets for connecting the speakers to the amp. Strip and tin the ends of each wire.
Solder speaker wires to +positive and -negative pins on audio output of the ST2012 breakout.
For an audio test, disconnect the 3.5mm stereo plug from the Pi and connect it to a device like a mobile phone.
Disconnect the jumper wires from Pi.
Connect the JST from the battery to the PowerBoost 500C and turn the slide switch on to power the circuit.
Play back an audio track, youtube video or anything that produces sound. You should hear some jams! If not, double check your wiring.
The ST2012 amplifier doesn't have an LED indicator, so you'll need to use a multimeter to check your connections.