利用MATLAB 读取一个WAV 文件，并画出其波形图。将此WAV 文件的信号幅度衰减一半后再存为另一个WAV 文件，并画出其波形图。-Using MATLAB reads a WAV file, and draw their waveforms. Signal amplitude attenuation half of this WAV file and then save it as a WAV file to another, and draw their waveforms.