Need some advice about audio (...and maybe general build advice.) Help save our build!

[infinitLoop]

it sounds like you are using both the left and right channels? (you can also do just mono and pick one channel) a ground should be going from your pi to the ground on the pot, then to the ground in on headphones, audio ground on the amp, and then to the negative speaker.

pin 18 is pwm0 by default, but 13 is pwm1 by default - if you use 19 as pwm1, you'll need to set it to an alt function.

https://pinout.xyz/pinout/pin35_gpio19

rodocop's diagram works:

viewtopic.php?t=3694#p40529

if you are foregoing the filter, just go from the pi to the pot.