Snape told Voldemort, he didn't know that he [Voldemort] wanted the Sorceror's Stone. Snape thought Quirrel was unworthy of the stone.
Just because Voldemort possessed Quirrel doesn't mean that he had access to all of his thoughts. It's unlikely that Voldemort, who was convinced that Snape was a loyal servant, would have gone looking for information that proves he was not. The one tremendous mistake that Voldmort makes throughout the entire series is assuming that Snape is loyal.