MANY, many people will offset audio levels while on set to avoid having the audio peak. For example they'll set channel 2 to be lower than channel 1.
This way when the audio does peak, it'll peak in channel 1 but not in 2 so you have clean audio. It's a great zero-cost way to have a little insurance that you have clean un-peaked audio. But it's a pain to clean up in post if you don't know about it before hand.
Tip #1: Listen to the footage with a decent pair of headphones.
An edit I'm working with now has the audio levels all over the place. My guess is that when they were shooting the cameraperson adjusted the audio levels on the camera when he or she remembered to; it's not consistent at all.
Here's my quick fix for it…after it's been dropped into a timeline. Ideally you'd only set one target audio track in the Timeline then center and boost from there but if your assistant didn't check the audio with headphones before hand…
In this case I only need channels 1 & 2.
1. Highlight the clip with the offset audio levels. In this case Ch1 is GOOD Ch2 is BAD.
2. Hit Option + L which is Toggle Stereo Pair. You'll see the "bow ties" on the clip vanish.
3. Hold down Option and click on the BAD channel, in this case Ch2. This lets you highlight just one of the stereo tracks.
4. With the BAD track as the only thing highlighted hit Control + B for Toggle Clip Enable. This will un-enable that channel. Disenable? Nonenable?
5. Hold down Option and click on the GOOD channel.
6. Now, hit Control + . (period). This will Pan Center the audio track.
7. If you need to boost it's level you can:
A: While it's still highlighted hit Control + + (plus) or - (minus) to bring it up or down 1 dB at a time, or use Control + ] (right bracket) or [ (left bracket) to bring it up in increments of I think, 3 dBs at a time.
B: While the clip is highlighted hit Option + Command + L to adjust Levels… (Unless you know the dB of the clip as is I suggest using "Absolute" here)
C: Head to Modify > Audio > Apply Normalization Gain (this does not have a key command by default, unfortunately) This will set the loudest portion of the audio to what you tell it here.
Be aware that if you try to set the audio tracks back as a stereo pair via Option + L after you make adjustments you'll undo most of the audio work you just did. I just leave them un-stereo-paired.
I prefer trying these in this order as usually option A works for me.