We prove an extension of Milnor-Wood inequalities to a geometric situation. We study representations of the fundamental group of a compact manifold into the isometry group of a product of rank one spaces of the same dimension and show an upper bound on the volume of the representation. When the target group is the isometry group of the real hyperbolic space, we show the constance of the volume under deformations using the Schläfli formula and deduce a new and simple proof of a result of T. Soma; the result is that there are only finitely many closed hyperbolic three-manifolds dominated by a given closed three-manifold.