Can't detach audio from .m4v movie clip
I took a video with a camera and want to replace with audio track from a digital audio recorder. I've imported the movie clip (.m4v) but the "Detach Audio" menu option is grayed out and can't be selected. Help!
iMovie 10.0.9
Yosemite 10.10.3
Mac mini, OS X Yosemite (10.10.3)