Combine, Sound Enhance, and Reduce Background Noise of 4 MP4 Media Files

Dear Freelancers, I have four MP4 files from my colleagues presentation, I need the foreground sound of the presentation enhanced, i.e. to make the sound more clearer (amplified I suppose) and all other sounds in the background dampened, except where the professor and anyone else asks questions--all sounds that belong in the foreground must be enhanced. I also need the MP4 files combined, i.e. edited together into one stream. Thank you for applying for the post, do not bid more than $75 please. I need someone capable to do this, it shouldn't be too difficult or time consuming for someone who already knows what they're doing. Good luck on the bidding!