Hi, kglea,
Possibly the playhead is located over a clip that does not have audio, in which case the Detach Audio option would be grayed out.
Try control-clicking on the video clip that has the audio embedded in it, and then click on Detach Audio from the drop down menu.
Usually deleting preferences cures problems of this nature. So if still no luck, try the deleting preferences procedure again, and afterwards reboot and reopen iMovie.
Also, you can try putting the clip into a newly created project and see if the Detach Audio procedure works there.
-- Rich