R key using iMovie

You cannot expand the duration of a sound clip beyond its original recorded duration (unless you slow down the clip). However, if the clip had already been trimmed to a shorter duration, then you can expand it back out by dragging the right edge of the clip. You don't need the R key, as Ian explains.

-- Rich

The R key or Range Tool is only for selecting a range in a clip like in the screenshot below where I wanted to lower the volume of that selection of audio.