If you have a mice infestation or even just one little mouse inside your walls, you're likely going to hear some scratching at some point. The only way to stop this scratching is to get rid of the mice. Setting one trap is not going to solve the problem because another mouse can just get in, and you may have more than one mouse within the walls. Additionally, kill traps are inhumane, often ineffective and unsanitary. You need to employ broader solutions to stop the mouse that's scratching inside your walls.
Set up humane traps for the time being. These traps are available in local hardware stores and home improvement centres. As you catch mice, release them outdoors far away from your home. Kill traps are unsanitary and can spread bacteria and disease.
Seal up any areas around your home where you believe the mice could be entering. Remember that mice can fit inside very small areas so be vigilant.
Remove all food sources for mice. Food is the only reason mice will come into your home in the first place. All food items in cupboards, drawers and pantries should be stored in glass containers that mice cannot chew through. Be sure to clean up all crumbs all the time; crumbs make tasty meals for mice.
Use fragrances that repel mice to help keep them away and out of your walls. Peppermint oil, mint plants, bay leaves, mothballs, onions and ammonia can all deter mice.
If you cannot find the areas of your home where mice are entering, a few sprinkles of baby powder may reveal their tiny foot marks.