Well, something has happened because today I can play the files in Media Player.
I emailed Audible and explained I was having trouble and that yesterday I got a message saying I had exceeded my activations and needed to apply to Audible to have them reset. Because I have had so much trouble burning CDs on my PC I tried downloading the files onto my laptop with XP and burnt them that way. With this and that I had exceeded the allowed actions. I received an email today stating that the activations had been reset and hey presto! today I had success.
Who knows the mysterious workings of the computer world.