Probably unecessary to buy, since the voice recorder in the iPhone (in my experience at least) is quite good. Instructions from the site:
For best results:
– Use good quality headphones (heart sounds are often too deep to hear using the white earphones).
– Press the microphone in the bottom of an iphone 3G to your chest. A good place is the apex of your heart (below your left nipple).
– Place the microphone directly against the skin, not through a shirt.
– Take the iphone out of its protective case if you use one.
Via: iStethoscope Pro