Your sound is an Event, it helps to make it a Stream when lip synching on the timeline. Go to your sound on the timelime, highlight the keyframe that it starts in, by clicking on it. Then go to Properties panel, Sound section, Sync: Stream. (It was probably on Event). Event will play a sound without stopping. Stream will allow you to scrub along on the timeline and you can focus on sounds that you can match your lips to.