Low frequency Raman scattering was measured for glassy As2S3 samples, prepared by different quenched conditions. It was found that the boson peak position shifts to lower energy with increase of the initial temperature of the melt and with increase of quenching rate. The effect is interpreted in terms of the dissociation reaction of As2S3 at high temperature. This leads to softening of the elastic modules and a decrease in the sound velocity.