The Sample Rate setting in the Hardware Setup dialog determines the default sample rate when you create a new session. This setting is available only when there is no session open. Otherwise, the current session sample rate is displayed, but cannot be changed.
With Avid HDX, Pro Tools|HD, and HD Native hardware, you can change the default Sample Rate in the Hardware Setup, or in the Playback Engine.
On Pro Tools, you can only change the default sample rate in the Hardware Setup or using the control panel for M-Audio and third-party audio interfaces.