Fixing too loud audio in iMovie 10.0.2

I shot a video after upgrading my firmware in my Canon 7D. I used the same lav mic I've used many times before and never had a problem with it, but this time the audio came out alarmingly loud, which I didn't discover until I played it back on the computer. It's like there's a buzz to the voice. You can understand what they're saying, and if I reduce the volume there is still that "buzz" type sound. Is there a way to fix this in iMovie? I dodn't know how to send a soundtrack so you can hear it.

Thanks in advance for the help!

No way to fix that in iMovie. You can separate the audio from the movie, take the sound into an audio editor and see if you can fix it (hint: izotrope's declipper plug in MIGHT help you -- costs $$), then put the fixed audio file back on the video and hope you get the sync-up spot on.

There's also overdubbing new sound over what got all distorted if you don't want to do that -- any chance of getting the participants and wild sound to do a little ADR?

Fixing that is going to be difficult at best.
As far as the original cause, does your camera now have a manual audio levels adjustment where it didn't have one before the update? If so, that is where your original problem lies-- you need to bring the levels down.

Yes, the camera now does. The lav mic always worked perfectly before I did the firmware upgrade. Never a problem. I have done everything possible and even tried some other "fixers". The client does not want a re-shoot and feels it's good enough with my "adjustments". If I would have used the shotgun mic it would have been fine. It's embarrassing to me, but gotta go w/ the client.

Live and learn to do an upgrade and then completely test before shooting. I've done this for 30 years and am still missing things..... hehe

I've now adjusted the lav mic and all works fine............. better late than never, I guess.