If the box is unchecked, try checking it. Also, instead of selecting the audio clip and clicking on the loudspeaker icon, try selecting the video clip above above the audio clip and then clicking on the loudspeaker item. See if the box is checked or unchecked, and try checking and unchecking it. If that does not work I don't know what could be causing it. Does it duck down when you put a different video clip above it?
You can try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your project. See if the diminished volume issue is resolved.
Also, possibly the issue is confined to just this one project. Try doing an Edit/Select All, Edit/Copy, of the media in your project and then paste the media into a newly created project. See if that solves it.
All else failing, as a workaround you can place your cursor at the beginning of where the wave forms duck down and, while holding down the "r" key, press down on your track pad and drag through the audio clip to the place where the wave forms show increased volume. The range that you dragged through will be outlined by a yellow box.
Now put your cursor on the horizontal volume adjustment line and pull it up to raise the volume for the selected range, to even it out with the volume of the rest of the clip.
As for other video editing programs, Final Cut Pro X is reported to be good, though more of a learning curve. Unfortunately it costs about $299. If you open the online store app you will see several less expensive video editing programs offered, some of which have free trials. I've not tried any of them so regrettably I can't give you any recommendations.
-- Rich
