I found 8 times was the best for me. Faster than that I seem to have no real benefit when listening to an audio presentation. It was as if I was listened for the first time and had no real connection to the information. After listening at 8 times I did have the feeling I've heard it before and was game to hit the fast forward button without fear of missing to much. Useless to try fast forwarding on a CD player with an anti shock delay since they take some seconds to reconnect the sound.
Windows media player has a little button at the end of the slider with >> click on that 3 times and it lets you speed up the playback up to 5 times. Saves hunting for the playback speed option and you might want to try it at this speed which I think you can notice something with. At the faster speed it seems to become harder to see any point. I think the benefits are rather obscure so why add even 1/16 to the listening time? In an hour recording that's adding 3.?? say 4 minutes when you consider that you physically have to start the recording a second time at normal speed.
Most people can listen to a recording while doing something else but when they start speeding it up to say 2 times then they have to sit back and concentrate on listening.
Prelistening at superfast speeds might prove more benefical to people who have difficulty listening while doing something else. If adding 5 minutes to the overall listening time allows them to be working on something else while listening then there is benefit.
This is one area where the experience is purely subjective you can only try to check it out for yourself.
Alex
[This message has been edited by Alex K. Viefhaus (edited April 01, 2005).]