Copy paste section of audio iMovie 10

This seems so simple but eludes me. I've got a clip I want to replace a section of audio with fill/copy from another section.

After detaching the audio I'm able to R/Drag to select my segment of source audio and COPY.

But - PASTE doesn't work no matter where I place the cursor.

I would think I'd see a new audio track appear in the detached audio area (green) but nothing.

Beginner stuff but it makes no sense to me... Pic attached of an example of how it looks - all I wanted to do was fill the "muted" areas with ambient copied from another area (for instance)