  We consider the efficient numerical approximation of acoustic wave
propagation in time domain by a finite element method with mass lumping. In the
presence of internal damping, the problem can be reduced to a second order
formulation in time for the velocity field alone. For the spatial approximation
we consider $H(\mathrm{div})$--conforming finite elements of second order. In
order to allow for an efficient time integration, we propose a mass-lumping
strategy based on approximation of the $L^2$-scalar product by inexact
numerical integration which leads to a block-diagonal mass matrix. A careful
error analysis allows to show that second order accuracy is not reduced by the
quadrature errors which is illustrated also by numerical tests.
