The unsub watches his videos of the victims on his computer. Whenever he fast-reverses or fast-forwards the footage, we hear the audio sped-up. This wouldn't happen unless he was watching it on a videotape with a linear audio system (such as early VHS) on a really old machine (that doesn't mute during searches). Considering that he's watching it on a computer, and that he shot it with a modern tapeless video camera, odds are, he's not playing a tape. In other words, we shouldn't hear any sped-up audio when he fast-searches the video.