My first suspicion would be the sound card and issue electrical and mechanical. (Dirty connection in jack?)
Do you have any other audio software on the machine. If so, try recording with it and see if it also has lost its vigor.
I am not aware of a gain control in the Audition software. There may be a menu that takes you to Windows Mixer controls, but they are part of the sound card drivers, and how the sound card interfaces with Windows. (Oooops. You are on Windows? Yes?) My AudioPhile 24/96 allows no control of recording volume. You have to have external ability to control what is going into the input jack.
You say you cranked up the gains. Does that mean if you turn down the input/recording volume on Windows Mixer, the sound being recorded gets even softer?
Some drivers hide a "check box" on the Windows Mixer screens to "Reduce input by 20db". Make sure you are not over looking one of those.
There are usually two controls in Windows Mixer that affect the level: the LINE IN or MIC IN control, and then the MASTER level. Make sure you are not ignoring one while trying to save the world with the other control.
Do you have any equipment the sound is going through on the way to the sound card that could have been changed? My mic preamp was incapable of creating enough signal to drive my sound card so I have a small two channel mixer hidden under the desk that supposedly never gets touched. I forget that it is there. I think my stinky stocking covered feet brush up against a control from time to time and I have to go through again and "stage the gain" all the way from mic to computer all over again.
And one final thing to think about. I once damaged a sound card by feeding in such a strong signal that some little solid state component in the sound card became damaged for life. I'm not sure how you would determine that was the cause in your case.