B, the installation of noise monitoring devices in the classroom can know the indoor noise level, not be able to attenuate the noise; therefore, B error;
C, will be turned down the stereo, is the source of the attenuation of the noise; therefore, C is correct;
D, the road sides of the construction of noise walls, which is the transmission of the D. Building soundproof walls on both sides of the road is a way of isolating and absorbing noise in the transmission path; therefore, D is wrong.
Therefore, choose C.