I assume you are using Audition 3, not an earlier version. You need to tell us which operating system your machine is using and whether you have installed anything like asio4all.
If you are on Audition 3 and Windows 7, and your machine has a jack labelled LIne In (I have come across a machine recently with a Line In option in Windows, but no actual input!), you should select Audition Windows Sound as the input in the Edit->Audio Hardware Setup in Auditioon, then via the Control Panel button check the DirectSound input port is selected..
After this it should just be a matter of setting the recording options correctly in Windows Control Panel, Manage Audio Devices. You should plug something in to the Line Input. This ensures that "Jack Sensing" in the OS isn't disabling something, then via the Recording Tab, ensure that Line in is enabled and selected as the default.
Let us know if this helps.
PS Release asio driver in background should be unchecked, but I don't think it will affect this.