problem for sound card or matlab?

When i use the simulink box, if i use the microphone only(not use the
speaker), the process for the from wave device and to the scope is in
the normal, but if i generate a sine wave to the wave device and see
the scope, no matter i set the microphone to how far refer to he
speaker, the scope also represent the magnitude as the magnitude i
set in the generater. How can I see or receieve the variable
magnitude in the scope? Please!