Why does the overall noise level changes (new resonances) when I change the nominal speed and voltage, although I keep the flux constant?

This shows an example of overall noise change when changing the nominal point:

Example of overall noise change when changing the nominal point
FAQ 2.3

This is due to the synthesized sonagram assumptions - normally everything below the speed marked with the red dashed line should not be considered, because the spectrum does not go up to the maximum frequency you have specified.

You should see a WARNING message on that in the command line. The reduction of the nominal speed (second graph) allows you to include more harmonics in the spectrum, then the overall SPL level changes. If you want to calculate the vibration and acoustic noise at variable speed using synthesized spectrogram assumptions, run the same analysis starting from low speed (e.g. 50 rpm in this case) to have all the resonances above 50 rpm right. All the peaks above the nominal speed should not change with the nominal speed if you keep the same flux level.