Possibly the audio was recorded at a slightly faster or slower speed than the project frame rate. Expand out your timeline with the slider located above and to the right of the timeline. Then control-click on the audio clip in the timeline and, from the pop up menu that appears, choose "Show Speed Editor". That will cause a speed adjustment handle to appear in the upper right hand corner of the clip. Drag the handle to the left to speed up the clip (raise the pitch), and to the right to slow down the clip (lower the pitch). See if that cures it.
Another thing you can try is to delete/reset preferences. To delete/reset preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open
in a new empty library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project. Now see if the pitch issues are resolved.
-- Rich