The audio files which were created on your first "Edit in Adobe Audition" must be saved on your system somewhere. Simply open them in Audition, do the necessary editing and save them. Next time you open your Premiere project, the new edited files will be present in your sequence.
Note that this editing in Audition is destructive, so you may wish to archive the original Audition edits before working on them.
Your right the files are there but all as separate wav files that were created when the Audition project was first created and I have the original Audition timeline that was created when I first sent the project back to Premiere. What I can't seem to figure out though is that when you send the project back from Premiere to Audition the new audio tracks that were created don't link back to the originally extracted wav files or the Audition Project that created them. Instead Audition creates a new wav file for each complete track (for those tracks that came from Audition first time round). However if you click on one of the single track clips from the original Audition to Premiere round trip and choose edit original it opens the original Audition project with all the clips in that created. Which isn't the behaviour I was expecting I was hoping for something a bit like the dynamic link relationship between after effects and premiere so if you have round tripped your audio any further changes in premiere are reflected in the audition project or a copy of the audition project. Is there away of doing this?
As I said, it is the "extracted" audio files which embody your original Audition edits, and if you wish to do further editing, it is these which you must open in Audition and re-edit.
Alternatively, you can if you wish, R click on the Extracted file in Premiere and select Edit in Adobe Audition, and you will then get an Extracted Extracted file in your Premiere project, which you can edit in Audition.