It's basic two-stage amplifier. Microphone picks-us acrostic signal from stethoscope head, this signal is amplified twice to reach levels usable in regular headphones. First stage has fixed amplification, second has a potentiometer in its feedback loop for sensitivity adjustment. Design works with voltages 4,5V-12v, filter capacitors need to be properly selected for voltage You choose to use. For tantalum please consider 20% voltage derating and for mlcc look-up capacity derating curve. If this design oscillates it is up to those big value capacitors to attenuate it. Keep cable run to the microphone capsule as short as possible, twist the wires and make them same length.