Question:
Can I use a stethoscope (for fun, not for health reasons) on shabbos to hear my heartbeat or is that a problem of hashmaas kol?
Answer:
It is not an issue of hashmoas kol because you are not making a sound, the stethoscope is only letting you hear the sound that the heart is making, Besides no one can hear it besides the person who has it in his ears.
