Maryland Code, Transportation § 22-401

(a) Every motor vehicle when operated on a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no horn or other warning device may emit an unreasonably loud or harsh sound or a whistle.

(b) The driver of a motor vehicle shall, when reasonably necessary to insure safe operation, give audible warning with his horn, but may not otherwise use the horn when on a highway.

(c) No vehicle may be equipped with nor may any person use on a vehicle any siren, whistle, or bell, except as otherwise permitted in this section.

(d) It is permissible, but not required, that any vehicle be equipped with a theft alarm signal device that is so arranged that it cannot be used by the driver as an ordinary warning signal.

(e) Every emergency vehicle shall be equipped with a siren, whistle, or bell capable of emitting sound audible under normal conditions from a distance of not less than 500 feet. However, the siren may not be used except when the vehicle is operated in response to an emergency call or in the immediate pursuit of an actual or suspected violator of the law, in which latter events the driver of the vehicle shall sound the siren when reasonably necessary to warn pedestrians and other drivers of the approach of the vehicle.