Volume varies with text to audio

[prototyperspective]

I'm using this wonderful software to create spoken Wikipedia audio files of the latest version of articles as described here.

A problem is that the volume is not consistent. For example in this file, it's louder towards the end than at the beginning. I don't know if it also happens in this file but sometimes the volume changes so quickly one notices it a lot and even if it's not such a sudden change, the audio volume should stay consistent. At worst case the audio volume could be normalized using some tool but there probably is some issues that causes it to generate the audio with different volume levels.