This is most likely because you are using on-board audio and the settings in Windows (7?) control panel are wrong.
Go into Windows Control Panel, Manage Audio Devices, and in both the Recording and playback tabs, go to Properties -> Advanced and set the default sample rate the same in both.
With some audio devices, eg Realtek, there may be other audio Control Panels as well as the basic Windows one. You may find they need setting separately, or not depending on what the designer of your system had for breakfast (NB that's a joke).
I think the same may apply if you are using a more professional interface in non-asio mode.
NB Make sure you set the default sample rate to match the files you are working with. Windows 7 has a built in non-adjustable and bad sample rate converter and will use it if you are not careful.
This might also occur if you're trying to use different audio devices for input and output, for instance a USB Microphone to record, but your onboard audio output for previewing. As WildDuck mentioned, these devices need to be configured to the same sample rate in order to play nice together. Once you decide on a sample rate (44,100 or 48,000 are the most common selections) you'll also want to configure Audition's internal sample rate in the Preferences > Audio Hardware... screen to match.
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