Con sordini means "with the mutes". A mute is a device fitted to a musical instrument to alter the sound produced: by affecting the timbre (or "tone"), reducing the volume, or most commonly both.
Beatrice Carford is spending the evening with her family and friends rehearsing a musical performance for the upcoming show week concert. The rehearsal is interrupted, however, when her niece June declares that she longs to leave their provincial town and see the world, just like her older cousin Harriet. The group begin to reminisce about their childhood memories of Harriet, who has become a successful and worldly travel writer. Suddenly, Harriet herself arrives at the house to pay a quick visit in between flights. She doesn't have much time to spend with her old friends, but it might be just enough to uncover a long-concealed secret that could turn Beatrice's life upside down.