Here's what the Help File Says:
You may enter any sample rate directly, or choose a common sample rate from the list. Note that most sound cards support only certain sample rates. Certain combinations of sample rate, channels, and resolution may not be available on your system. To see the capabilities of your system, check Options > Device Properties. Although you can create and edit any sample type with Cool Edit Pro, your sound card may not be capable of playing it properly.
Hope it helps,
Chris