There is a character from the wonderful 1965 movie "A Thousand Clowns" credited as Sandra, but she pronounces it Sondra, and so this is how I have always imagined it spelled. She is a beautiful, exuberant woman described by the main character Murray as "a lover of things and of people" and that she should "have all the gratitude" she can that she is "capable of embarrassment and joy" and is "a marathon crier." As a result, I associate this lovely name with that movie and her attributes.
One of the main characters in the novel "An American Tragedy" by Theodor Dreiser is named "Sondra". I always much preferred this name to the far more common "Sandra", it makes you think more of the sun than the sand. It sounds special and pretty. It even looks better written down.