So I followed the steps in the adafruit gameboy build instructions: 

https://learn.adafruit.com/pigrrl-2/overview

^ This is a brilliant way to bypass the need for using GPIO pins for an audio output.

Is there another way to do this??

Brought these components:

Power Slide Switch [ADA805] - https://thepihut.com/products/adafruit-breadboard-friendly-spdt-slide-switch
PowerBoost 1000 Charger Board [ADA2465] - https://thepihut.com/products/adafruit-powerboost-1000-charger-rechargeable-5v-lipo-usb-boost-1a-1000c
Silicone Wire [ADA1877] - https://thepihut.com/products/adafruit-silicone-cover-stranded-core-wire-2m-26awg-red
Audio Board [ADA2130] - https://thepihut.com/products/adafruit-mono-2-5w-class-d-audio-amplifier-pam8302

Miniature Speaker [117-6046] - https://uk.rs-online.com/web/p/miniature-speakers/1176046/

Lipo Battery (2000mAh) [BAT0005] - https://shop.pimoroni.com/products/lipo-battery-pack?variant=20429082247

Now its time to start building...