Playful and witty, Noël Coward’s songs boast a lightness and effervescence that are immediately disarming. That’s why the expressions of love and longing in songs like “Sail Away” and “Someday I’ll Find You” have the impact they do. As a gay man in mid-20th-century Britain, Coward had to maintain a certain discretion in his life and his work. Yet as so many interpreters of his songs have proven, the contents of his secret heart are always clear.