If you are using different video clips or audio tracks in your project, you may get different playback volumes for each file.
Audio normalization is usually applied to make audio levels consistent throughout a video. It helps even out the audio to produce a better playback experience.
CreateStudio Pro gives you the option to turn this feature on/off.
When you're done editing, click on the Publish button > turn the Audio Normalization ON/OFF by using the toggle switch in the right panel.