Suddenly the Windows firewall detected Zoom and seems to have disabled sound -- although the mic from the USB headset was audible to the other person.
I'll have to update this after going through all possible settings, but it's just a forever problem with sound on tablets seemingly.
I unplugged the headset, still couldn't get sound. On the USB headset, adjusting volume caused the volume indicator icon to go change accordingly.
This happened immediately after windows defender asked whether to allow zoom (to which I said yes).
It's a surface 3, but: