Combining Clips from Difference Videos and Issues with Audio

Hi Folks,

I have been working on a project where I am editing clips fetched from different videos to make a final video with funny outcome. The problem is that the speaker in all the videos is speaking different environments. One video has him speaking in an auditorium, one in a room, one in a public rally, one in an open ground where there is lot of background noise. The resulting video has so many audio glitches. The audio sometime explodes and sometime it is very low and there is always a "transition" sound when the next combined clip is played. I tried using Essential Sounds which equalizes the volume for me but the transition sound still remains and the fact that the speaker is different environments makes the end result very poor.

Does anyone have any suggestion on "standardizing" volume and sound in such case?