Improving the sound quality of recorded video

I want to improve the sound quality of a lot of videos that exist in the repository of my company. There's a lot of white space in the videos and, at times, the voice quality of the artists in the video is not that great.

Can someone point me in the right direction or help me with this.

I am based out of India and can offer some monetary compensation for the help.

Audio stream is available in mp3.