I too have the av500 - what a machine!
I found that alhough my videos were AVI files they would not copy across to the Archos. Bt using the SYBC in Windows Media Player the files were changed to a form which was accepted.
Have you updated the machines OS - it should be 1.3 something, not, as mine was, 1.0.
I had trouble recording programs from the DVD recorder/player ( live TV that is) and found that, for the sound I had to use a svideo cable plus the 3 phone cable (using just the red and white phonos) to the docking attachment. The Dock was attached to the Archos, the power lead, via adaptor) to a power source. The phonos plug is of course for video and sound - I find that care has to be taken when inserting this plug into the dock - it's best to feed it in when a video is seen on the Archos screen - watching the sound level on screen whilst doing so - put the plug in completely cuts the picture - not far enough momo sound. Get it right by trial and error - recording a minute or so and then listen (by Headphones) to see it stereo recorded.