My 15+ year old heavily used Cardiology IISE began to sound muffled, but all parts appeared intact.  Cleaning had no effect.  On a guess, I replaced the ear pieces, tested the stethoscope, replaced the diaphragm, tested again, and each time the sound quality improved dramatically.  Its now superior to 2 older Littmann Cardiology stethoscopes that were not as heavily used.  The improvement suggest age related parts deterioration, regardless of use.  I used my own A2-P2 heart sound split as a test of functionality.