Set your audio board up in a pair of helping hands and heat up your soldering iron. Always solder with good lighting so you can see what you're doing.
Tin the long battery pads on the audio board with a little solder. Position the JST battery connector on the pads and reheat the connections while pressing the jack to the board for a flush fit.
Also solder the metal tabs on each side of the battery connector to the corresponding pads on the circuit board.
If you have any arm switches, combine their ground wires and an extra for the membrane button pad's ground. Twist the wires together and solder to GND on the circuit board. If you forget, there are other GND pads around the circuit board you can tap into later.
position your speakers at the edges of the kangaroo pocket and trim the wires with a bit of slack to reach your circuit board. Strip the wire ends, twist and tin the leads, then solder them to the speaker outputs on the audio board.
Premium jumper wires can extend the leads on the membrane keypad. Plug in a strip of five headers and strip the wires at the opposite end.
Twist and tin the leads with solder. The wire closest to the end of the strip with the button marked 1 is the ground wire.
Solder the wires to the numbered pads on the audio board-- you can set up which button makes which sound when you load the files.
Solder the membrane keypad's ground wire to a GND pad on the circuit board.
Lay out your circuit on the outside of the kangaroo pocket. Grab your battery pack too. Position the membrane button panel to sit at the inside edge of the pocket opening.
Reposition the circuit on the inside of the pocket and stitch the speakers in place using the mounting holes at the corners. I'm using a contrasting thread so you can see the stitches better!
Likewise stitch the audio board to the sweater using the mounting holes at the corners.
Peel off the adhesive backing from the membrane keypad and stick it inside the pocket's opening. You can optionally stitch it to the pocket in a few spots if it does't stick well to your pocket fabric.