easiest way to get rid of mice in walls?

Find where they're getting in first, otherwise you're just trapping the current lot. Gaps around pipes, cables, vents and under doors - mice need a gap about the width of a pencil. Fill with wire wool and then sealant, they chew through foam on its own. Then snap traps along the walls near the noise or droppings, baited with peanut butter. I'd avoid poison when they're inside the walls, they die in the cavity and the smell is grim for weeks.