Okay...if that hardware test didn't work then your problem is someplace before the signal ever gets to Cooledit/Audition.
Things to check:
You mentioned a mixer...make sure the sound is set properly there. There will likely be a meter or flashing LED or something to indicate proper sound, or you may be able to listen to the output on headphones.
Check all cables...make sure they're in the proper sockets
Check the Windows Mixer on your PC to make sure the proper input is selected on the RECORD menu. If you're feeding from a mixer this will likely be Line in; the test may have auto selected Mic in for you.
Basically you just have to be methodical and check each part of the chain in sequence.
Oh, and reading the manual is a GOOD thing for the future...but I doubt it would help on this one since the sound isn't even getting into your computer.
Bob