The best way to get rid of a dead mouse smell is to remove the body, and then allow air to ventilate the area. Should the dead body be inaccessible, the next best thing is to mask the odor with a disinfectant or an odor neutralizer.
Mice often die inside walls, making removal of the body difficult. If the body can be located by the sense of smell, a hole may be drilled in the wall, with the body removed through the hole. This requires repairs to the wall once the mouse is removed. A less expensive option is to use disinfectants and odor neutralizers until the mouse's body dries up. This may take in excess of three weeks.