This is from a fresh installation of Ubuntu 19.10, fully updated as of Feb-29-2020.
Sound comes out just fine from the internal speakers. If something is plugged onto the headphones jack, the internal speakers do not go on mute, and nothing comes out of the headphones. On Windows everything works perfectly fine.
According to this spec sheet, this laptop has the Realtek ALC294 chip.
I've tried disabling fast-boot and do a full shutdown and boot up from scratch, as it's been suggested on other posts I've read. No progress there.
Anything else I could try?
Full specifications from Asus can be found here.