Wouldn't hearing Snape's counter curse tip off Voldy to his disloyalty?

So, in the firts HP book, im assuming, i cant remember but its in the movie and seems quite important, so i think its in the book, Quirrell/Voldemort trie to kill Harry in his first Quidditch game. While murmuring his curse, Snape begins to chant a countercurse to save HP. Near the end of the movie, when Voldy and Harry confront eachother, Voldy says that he heard Sanpe's counter curse. Wouldn't that make Voldy aware of Snape's double-agenty ness and he would want to kill him, or does V want to keep Snape alive for his bidding, or what?