Difference between the Volume of the Audio and Video clips from the same recording

I have a problem that seems to have occured for other users of Storyline... to quote another user "When I import a video (.mp4), it plays louder than if I import the audio file (.wav) of the video directly. The problem now is that the videos have a different volume than the audios, which leads to unattractive side effects."

What can I do so that there isn't a difference in volume between the video and audio files from the same recording?

I am using Storyline 3.

Thanks,

Jo