First go to iMovie/Preferences and make sure that the Advanced Tools are enabled.
Now, drag the clip with the Video into the project.
Drag the clip with the Audio and drop it on the first clip at the frame where you want it to begin. A popup menu should appear. Choose Audio Only. You may then want to open Inspector for the video clip (by clicking the little gear icon at the top left of the clip) and going to the audio tab. Here you can turn down the volume of the video clip, so that all you hear is the audio clip.
Another way. Put the video clip that has the Audio you want in first. Then drag the clip with the video you want and drop it on the clip that has the audio you want. A popup menu will appear. Choose CUTAWAY.