This has to be a problem with the Windows 7 recording mixer or the Realtek drivers.
Is the Windows Mixer set to have "Stereo Mix" as the default input? If that doesn't appear and you are using a line or other input, is there a jack plugged into that input before you start Audition? Win7 has "jack sensing" which can cause problems with pro audio.
If these are both correct, I'd try looking for different Realtek drivers.
I have a similar problem but is just when i hit record even with the mike off it records a hissss in the background. I updated from XP To WINDOWS 7 32 im using adobe audition 1.5 and is really frustrating any help????? when I had in xp everything worked fine just when I updated it happened!
Download the new Audition trial, make sure it works on your system, and then spend the $99.00 and move on.