A:Start singing?
Practice breathing exercises while you walk. There are different exercises (including some used by the U.S. Air Force) that can focus your mind on the breathing so that you won't unconsciously hum. One common exercise is to take a breath in for 4 counts, hold it for 4 counts, release it for 4 counts, and keep the air out for 4 counts. I doubt a person could hum while doing that one.
