Heart and lung sounds are lower frequency than most speech sounds. Most speech sounds are from 250 – 4000Hz, and many are higher. Heart sounds are 20-650Hz and lung sounds 70-4000Hz, most below 2000Hz , . These are lower than the audiogram usually tests.
If your hearing is too poor in the low frequencies, you will need an amplified stethoscope. If you only have a high-frequency hearing loss, you may be able to use a normal stethoscope.
Next decide whether you’re happy to take your hearing aids out when you use a stethoscope.
 These figures are from Cardionics Technical Information Bulletin E-7710. When I researched this, different stethoscope manufacturers quoted slightly different figures. Different models don’t amplify the same range of frequencies as others. Check out the technical specifications.
 Technical Considerations in Using Stethoscopes explains this in more detail and includes the frequency range of bowel sounds.