Like many good things in life, Sara Sawyer’s “yoga in the garden” class was born out of necessity.

What is it about music, particularly the songs we heard when we were young, that gives it the power to remind us so vividly of time, place, emotion? Tom Waldman, playwright, television host, and author, taps into this question in his highly entertaining new personal, historical, and political musical memoir, “Countdown: A Life in 20 Songs.”