In general this is what you must do;
Set sample rate of Cubase (project setup) and of audio interface (in the interface settings) to the same sample rate…before you start work!
In preferences/audio set the On Import audio files option to open option dialogue and when you import a new file it will ask if you want to change the sample rate.
As to your current project. What sample rate was your project originally set to and what rate are you exporting to?
And how are you playing it back? What is your audio device?
And did you record some audio in the project or are using pre-recorded imported audio only?