Normalize sound level

I found this app on Play Store https://play.google.com/store/apps/details?id=com.rebuildlabs.waver My device cannot install but I also read its description.

Waver is a free smartphone app currently available on Android that uses advanced track level analysis to automatically balance, adjust, and normalize all sound levels in your music library. While similar to store volume control apps, it aims to solve the problem of audio tracks in your library with varying volume levels. As a background program, it does not take up much RAM and is compatible with most music players.

I want to do the same in my application. For example, I am playing a list of audio that has a difference in audio levels, some too loud and some too low. How to control the volume automatically or normalize the sound level so that you can hear without a difference?