Trouble editing volume

I am having trouble getting a consistent volume in my Storyline project. The audio for some slides is louder than for others, and there is even variability within some individual audio clips. All of my audio was recorded from a microphone. When I change the volume with the audio editor, save the project, and publish it, the volume in the published project does not change. How can I get a consistent volume level throughout my slide presentation? Is using another software program the only option? If so, are there any good free programs out there that would work?