Welcome to iMovie Discussions!
I think it still works the same way as previously in iMHD5 (..I only have 5 in front of me right now..)
1 - Click on
iMovie at top-left then on
Preferences:
under 'General' tick 'Extract audio when using "Paste Over at Playhead"'.
2 - Select (click on) the still clip which you want to insert
3 - Click on
Edit and
Copy
4 - Click on your video clip in the Timeline and position the Playhead where you
want to insert the still clip
5 - Click on
Advanced and
Paste Over at Playhead.
That will keep the existing audio (..and will put it on a separate audio track beneath the video, unless you've already done that by 'Extracting' the audio..) and will lay your Still over the existing video, with the audio still continuing as before.
(There's also an item 0, which comes before all that, which is ..no, really; this really works, and you get so much more out of iMovie this way.. click on
Help at the top, then
iMovie HD Help, and in the Search field at the top right of the Help screen just type
Insert. (..That's a reasonable word to use, as that's roughly what you want to do..) ..And that should explain exactly how to do it. Same with other problems.)
And try the helpful
iMovie Tutorials and other informative items there ..and don't miss
Dan's excellent iMovie FAQ ..where nearly every problem and its solution is listed!
Good to see you here, though..