It's OK for the metal speaker frame to touch and make electrical contact with the ground plane that surrounds the plated through mounting holes.
But there are active traces that run underneath the speaker.
Those the frame should not touch. But they are sufficiently far away from the washers.
Given that you have a mismatched pair of washer thicknesses - don't put the like-sized ones diagonally opposite, since that would torque the frame.
Put them on adjacent screws.
I cut up old dark socks (after washing!) to make cheap "grill cloth"