I'm using CS6 and I attached external speakers, but they are NOT showing up in the Audio Channel Mapping. Can someone help with this??
But "line out" of what? On board soundcard, PCI plug in card or external audio interface?
Also note that some Microsoft operating systems have plug sensing hardware/drivers that can be either inputs or outputs, sometimes apparently at Microsoft's whim.
How many line-outs could there be? Just try it again and again until it works. If it doesn't, go to the preferences both in Audition and your computer, make sure everything is set, and keep trying the different line-out plugs. If it still doesn't work, then you're probably bat-sh** out of luck.
We still need to know what the OP is using for audio out of his computer and what operating system it is using before any useful advice can be given. At the moment we can only assume that it is a Windows PC of some sort and that he might be using an on board soundcard.