Raynec I'm sure there's some method to getting into BIOS/UEFI, but let's not go there. Let's forget that at least for now, because I just wanted to make sure that you haven't already been tinkering there turning off stuff, but obviously you haven't. So that's out.
From pictures of pixelbook I can see there's mute- button, upper row, fourth button from right. You're aware of this, yes? It is not pressed?
Also, Axios here seems to have more professional approach of troubleshooting computer problems so let's wait what this person has to say about the output of pactl info.
My (very little) educated guess is that, where it says default sink and default source, there shouldn't be device_null, instead there should be something indicating your default speakers.
edit. *device_null, I meant auto_null, or something... _null