Listening to lung sounds is such a variable experience. So many people talk to you while you have the stethescope in your ears, moan/groan/hum while breathing or even hold their breath. A lot of the time, they will take a deep breath and hold it. Or breath so slowly that you can't hear anything.
I have developed a habit of saying, "OK, can you take a deep breath, in and out." Sometimes I have to say, "Don't hold your breath, just breath in and out."
Just now, a patient was humming I Wish You a Merry Christmas
while I was trying to listen. Good humming, though.