Hi,
I did a test video and could not replicate your issue. I sped up a portion of the video clip to 20x speed. The audio clip below played normally through the sped up portion.
Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the audio plays normally.
Also, possibly your issue is confined to this one project. Try creating a new project and putting your media into it. Speed up portions and see if the audio below plays normally.
-- Rich