Gather up the parts we need for the audio circuit. We'll need four pieces of 26AWG wires, two slightly longer than the others. Go ahead and remove 5mm of insultation from the tips and then apply solder to tin them.
Connect Wires to Speaker
Heat up the solder pads on the speaker and remove the existing wires. Then, connect the new longer wires. Appy a bit of heat shrink to keep the wires tidy.
Connect Wires to Audio Amp
Secure the PAM8302 to a panavise or helping third hands. Tin the audio out pins then VIN and GND. Connect the speaker wires to the audio out pins and then the remaining short wires to VIN and GND.
Now we can connect the VIN and GND wires from the audio amp to the 5V and G pins on the PowerBoost 1000C.
Connect Audio Cable to PAM8302
You'll need to cut the audio cable short to a length so it can reach the audio jack on the Raspberry Pi - leave a little extra slack just in case its too short.
Remove about an inch of insulation using a hobby knife - be careful not to cut any of the inner wires or stranded wires.
The stranded wires are actually the ground. Twist these together to group them. Then, remove about 5mm of insultation from the red and white wires (these are the left and right audio channels). Tin all three wires (right, left and ground). Now we can solder the ground to the A– pin, and both left and right wires to the A+ pin on the PAM8302.
Alright! Now our audio circuit is ready for some testing. Power on the PowerBoost 1000C using the slide switch. Plug in the audio connector to your computer or mobile device and play some audio, music or anything that makes sound. You should hear it play through the thin plastic speaker.
I recommend turning up the audio gain on the PAM8302. Use a small flat screw driver to twist the tiny potentiometer located on the amp PCB. This will make the sound play loud as possible.
Cut the Fat
The connector from the audio cable is a bit excessive - it actually won't allow the Raspberry Pi to fit inside the enclosure as is, so we'll need to trim it down a bit.
Using a sharp hobby knife, carefully cut away from the back of the audio connector - like in the photo. We need to remove about 3mm of plastic from it. Cut it all the way down until you just barely reach the audio cable.