  This article deals with the weak errors for averaging principle for a
stochastic wave equation in a bounded interval $[0,L]$, perturbed by a
oscillating term arising as the solution of a stochastic reaction-diffusion
equation evolving with respect to the fast time. Under suitable conditions, it
is proved that the rate of weak convergence to the averaged effective dynamics
is of order $1$ via an asymptotic expansion approach.
