audio volume remain fixed

The feature that you describe would be called dynamic auto-leveling, and, if MPlayer doesn't implement that feature, that means you would have to parse and analyze files for volume levels, and modify them. I suggest you evaluate if the MEncoder offers any features for this.

And, why not start by looking at CodeProject resources for using MPlayer:

http://www.codeproject.com/Articles/176354/Simple-Mplayer-Front-End-in-C

http://www.codeproject.com/Articles/296490/Use-mplayer-as-our-Audio-Decoder