The length of the cable adds a negligible amount of delay. Most commonly the audio needs to be delayed because of the video lag in the monitor. (all LCD and Plasmas have delay in varying amounts, in my experience 1-2 frames) There are audio delay devices which can do this for you. Some higher end consumer receivers also have audio delay capabilities built in. I use an Onkyo for that very reason. If I’m not mistaken the frame offset in the SYSTEM SETTINGS>PLAYBACK CONTROL adjusts video and audio at the same time and is designed to allow you to sync an external monitor to match the computer screen playback.
Mark