Yep, there's real science behind that statement.  Sounds come in through the ears, but it's the brain that interprets them.  Nina Kraus of the Auditory Neuroscience Laboratory at Northwestern University suspected that the hearing systems of musicians should be more finely tuned than those of other people.  She tested that hypothesis by subjecting musicians and non-musicians to a challenge that most of us are familiar with - understanding what someone is saying in a noisy room.  The test asked the subjects to listen to a voice reciting simple sentences against an increasingly loud background of other conversations.  The musicians were consistently better able to hear the sentences than non-musicians.  Why?    
    Standard hearing tests showed that the musicians' ears were not any more sensitive than those of the other listeners.  But, what is different is that the musician's brain is trained to do a similar task - listening to his own instrument as many other instruments are playing - just like separating one voice from a crowd of voices.  Musicians were better able to pick out a particular voice in a din of voices the same way they pick out the sound of a particular instrument.
    Another study by scientists from Friedrich Schiller University in Jena, Germany found that musicians could detect harmonies that were slightly off-key even when they had lost much of their hearing.  Factory workers with similar hearing loss could not.  

Take the test.