Only works if slides don't omit important details or you are re-visiting the slides after having prior knowledge. Some ppts are more like visual aids (outlining important points in a few words) yet doesn't provide any detail. I enjoy both slides and audio. They compensate each other, provided that you have an articulate speaker... Whenever I read slides with proofs I rather have an audio stepping through the entire proof. Staring at proof or some data flow diagram doesn't convey enough to me.