You can quieten it down by putting a bit of electrical tape over the hole.
I just checked the dashboard of my Mk1 and there is no hole on mine. The sound just emanates from somewhere behind the instrument panel.
No, you have to remove the instrument cluster to get to the speaker. It's a small metal can on the rear of the cluster pcb on the mark 2, with a small hole in it. Not sure about the mark 1, I would imagine it's similar.
I would leave well alone too.