Currently, Voice over IP (VoIP) communication service is widely used mainly because its lower cost. However, the quality of a VoIP communication is lower than a fixed-line communication due to network degradations. In this context, the objective of this work is to evaluate the quality of voice communications that undergo degradations in the wireless interface, and how MIMO technology helps to improve the voice signal quality. For this, a simulator named WVoIPSim is built, in which different modulation schemes, an error correction coding and different noise levels in wireless channel are implemented. The resulting speech signal, at the receiver, is analyzed by the ITU-T Recommendations P.862 and P.563, in which P.862 is used as reference. Experimental results show that there is a high correlation between MIMO system configurations and speech quality scores. Based on these results, different MIMO configurations for different network conditions is proposed in order to guarantee a MOS score higher than 4.0.