The invention relates to a musical instrument having free reeds set into vibration by a flow of air generated by a supply (17) and capable of flowing from the supply in two directions referred to respectively as in and out, the instrument comprising: at least two boxes (2, 3) that are movable relative to each other by moving towards each other or apart from each other; a series of free reeds (27) mounted inside said boxes; and a valve actuator mechanism (31) where opening and closing the valves enables the reeds to be engaged or not engaged by the flow of air, the mechanism including in particular actuator keys (11) mounted on the moving boxes. The instrument of the invention includes means (5) for supporting and guiding displacement of the boxes relative to each other along a determined trajectory.