Linear PCM audio issue

Not good news, I'm afraid - it's really difficult, verging on impossible, to undo the effect of volume pumping. Your only real bet is to try to find a compression setting that will reduce the loud parts so that they are at a similar level to the quiet bits (this won't increase the pumping sounds in the background), but even then it won't sound good, as you'll lose all of the dynamics. A large part of the problem with treating anything like this is that you have no idea what the characteristics of whatever limiting is in the camera are - and even if you did, you'd have a lot of difficulty in mirroring that in reverse, anyway.