Exported audio sounds different than on timeline

[Walte58]

When I am editing the voiceover in Premiere Pro it sounds pretty good, but after I export the video to an mp4 it sounds like I'm speaking in a large empty auditorium. Why does it sound so different than when I am editing it and how do I fix it? If the audio isn't going to sound the same once I export it, the audio editors in Audition and Premiere become useless.

[RS3793]

I today had another issue:

My De-Noising was active, but it was not exported. I never had this before. After hours of research I set it from strong to weak noise cancelling (it is in german, so I am not sure about the very right names of the settings in englisch) then back to strong noise cancelling. All problems solved.
Everything was correct, all the time... It simply was not applied and you no way you could see it 😞