Why does the audio volume fluctuate on different slides upon publishing to HTML5?

I am using Captivate 8 on Windows 7. I have a project with 140 slides, most containing mp3 audio of recorded narration. I have normalized all of the files using Audition. I even pulled the files into the Match Volume tool in Audition after publishing and saving them in the "ar" folder within my published HTML5 project. However, still when I play the project back, some slides sound louder than others. This does not happen when I find the affected audio files and play them back outside the project. It also doesn't happen when previewing the project in Captivate.

I've found several posts about volume issues, but none with this specific issue. Any thoughts on ways I can correct this?

I thought of using the normalize feature inside of Captivate, but I'm trying to avoid it because 1) the source files are already normalized and 2) I have closed captions on every slide and would like to avoid having to redo those.

As a side note, I have also experienced this exact issue even using the text-to-speech feature in Captivate, in this same project and in other project files. The problem only appears after publishing.

Thanks in advance for your help,

Mark