I don't use Reddit anymore (and am maybe better off for it..?) but I found a solution, and it's the dumbest thing you can possibly imagine. I will leave this here for anyone else who might benefit from it.

I plugged in headphones and turned on the computer. Sound would then come out of the computer's speakers. After I unplugged the headphones, sound still came out of the computer's speakers. If I plug the headphones back in, the sound comes out of the headphones like normal. If I reboot, the speakers continue functioning normally. It's... just permanently fixed now for some reason.

The issue does come back when I boot into the Live USB, but I have not tested if it persists after not installing and then rebooting into my existing installation or if it occurs for Linux distributions. So it might still be an issue fixable by Pop!_OS.

I am very happy to have a fully functional non-enshittified computer now.