Assuming you mean a digital lowpass filter; this won't work. The ultrasonic input is saturating the microphone. In this situation, no amount of digital filter

Assuming you mean a digital lowpass filter; this won't work. The ultrasonic input is saturating the microphone. In this situation, no amount of digital filters can recover the desired signal (the speech). If there was a physical low pass filter (not sure if these exist) that was placed over the microphone, that would work