Apparently, and according to the manual, the original audio track is replaced by a new audio track with the changes. The only way to restore the original audio track is to delete the audiotrack (which is separate from the video track) and then load the original track, separate video and audio, remove video and move the audio "under" the video.
This only works this "simple" of course when you have not cut the video, otherwise you either have to replace the complete original and do the cutting again.
What I also noticed is that the (all!) newly generated audiotrack is stored in an AudioDenoise folder. Hence these file is separate from the project files. It looks like it is wise to pack and save the project, to ensure that the audio files are included and available to the project at later stage.
It may be worth it to submit a suggestion to CL, to enable an undo. I recon (because of the cutting and other changes) it will not be easy.