For those of you visiting this page because you're having this problem, what's happened is that Adobe, in it's infinite wisdom, has installed a second folder of program files when updating from 6.0 to 7.0 (or perhaps a smaller update). You need to eliminate duplicate folders so Premiere Pro understands where to look. I noticed that my Audition was all by itself in a second Adobe folder, so I moved its contents to the other Adobe folder with the rest of the program's files and made sure they sat squarely in the Audition folder where they once were. I then right clicked on one of the .sesx files and opened it with Audition and viola. I can edit Premiere audio in Audition again.
PS Adobe, I literally use this ONLY because the denoiser in Premiere Pro doesn't work for the first 2 seconds you play a clip. It takes away noise nicely, just never for the first two seconds after I hit the play button, as if it's trying to use that time to figure out what to remove. If it worked correctly, I wouldn't hear the noise go away in my videos after the first two seconds of a clip and I wouldn't have to export to Audition and waste hard-drive space and time.