Well, back then it was never a "proper" part of the CS bundle. Adobe bought a company called Syntrillium that made Cool Edit. At that point, it was up to Cool Edit Pro 2 and they did a rushed re-branding exercise to release Audition 1.0 which was identical to CEP2 except for the splash screen and a simple command to switch between Premiere and Audition. After that, they dropped Audition from CS and provided Soundbooth instead--a much simpler audio package developed in a rush by the Audition developers.
In the intervening time, Audition continued to be developed and went through AA1.5 (minor changes to AA1.0), AA2.0 (a big re-write released before it was finished to match Adobe scheduling and fit in with Soundbooth which was being developed by the same people at the same time) and then AA3.0 (another major change and a very good, stable package). All those were released as stand alone software, not part of the Creative Suite. They were known by the Audition release numbers, not by a CS prefix.
The first version of Audition to be known as CS(x) was released in January 2011 and known as Audition CS5.5. This would have been Audition 4.0 except for the change. In May this year they came out with Audition CS6 which is the current release (and a darn good piece of software.
Anyhow, although CEP2.0/AA1.0 was sketchily linked to Premiere, it wasn't a "proper" part of the Creative Suite (I don't know which number was current at that time) and AA then went through some years in the wilderness, supplanted in the CS by Soundbooth. None of those versions had "CS" numbers and, instead followed the Audition version sequence. The first one with a CS release number was CS5.5 in 2011.
Can you tell I'm a long term Cool Edit user who lurks on the Audition forum (but has little to do with the rest of the Creative Suite?