Will an audio CD set off a metal detector such as in an airport or prison?

No. They are made of a layer of plastic glued to a layer of aluminum
(aluminum is  non-ferrous). Aluminum is however conductive, so there
is the posibility of the magnitic field in the detector generating a
current in the disk which would in turn generate a magnitic field that
would trigger the detector. Although I have never heard of this
happining.

More on metal detectors:
http://home.howstuffworks.com/metal-detector.htm

Note: If you are planning to sneek some CDs past the detectors in your
local music shop, don't. Those towers aren't metal detectors, the are
guilt detectors that can pick up on the brain wave patterns that
eminate from someone who is feeling unconsciously guilty for stealing
something.