I just re-read the original post and I caught the fact that Harlan was looking at changing the sample rate of a "session" rather than a "file". The session sample rate option was "grayed out" because you were attempting to change the file sample rate rather than the entire session sample rate in Multitrack. The session sample rate is different than a file sample rate. Follow Ryclark's suggestion about F11 to change the sample rate of a single file.