I hear a mouse in my wall?

Try to find the way it got in to start. remember mice do not need a large hole their rib cage collapse so they can get into tight spaces. I would look under the sinks possibly where the plumbing is coming out. Next you want to pick up bait traps and put them near openings to draw it out. The last thing you want is for it to die in the wall. Just like any Other animal it body will decompose. That can lead to a terrible oder. You can pick up snap traps at your local dollar store or any hardware or home improvement store. Peanut er makes a great bait.