Digital Stethoscope

This is modified from www.thingiverse.com/thing:266747. I put an earphone inside to record sounds. It works for pulmonary sounds but not for cardiac auscultation. Likely because cardiac tones have low frequency (50-70 Hz) and the standard mic of an earphone is not enough sensitive. So if any 'sound technician' can help please suggest electronic and special mic to make this digital stethoscope work for heart examination too. Thanks !!