I know this is probably holy war territory, but I'm hoping for some
interesting ideas.

Starting with an audio CD I can take a "bitwise" image, using something
like Nero. I can then write that image to a writeable CD. I can take a
bitwise image from the writeable CD and compare it to the bitwise image
taken from the original CD. They are identical (with the possible
exception of a time stamp inserted in the image).

My brother-in-law claims to be able to hear the difference between the
original and the copy, but only when the copy was written at greater
than 4x speed.

What plausible causes exist for the audio being different when the
individual bits being read off the CD are not? They may arrive with
subtly differing timings, but the sequence is identical. I'm sure the
timing of the bits varies every time a CD is played, due to varying
rotational speeds of the CD.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk