There should be a way to set the input level in the audio software. Set the mic level to the lowest in the computer's mixer, or install an audio attenuator between the mixer output and the input.
What you want is to set the input to "line" level. Sometimes you can do this in the audio software or computer audio mixer.
Double click on the speaker icon in the lower right hand of the desktop. There should be a "mixer" button to bring up the mixer and adjust each level separately.