nenea dani, this is all quite interesting, and I commend your efforts.
The various dc/dc on the soundcard I use (EMU 0404 USB) causes a known corruption within the 96kHz bandwidth of its measurement spectrum, and users identified a way to suppress that by re-routing a signal line away from a dc/dc internal to the soundcard. For the audio work I do, which certainly extends to 96kHz, and beyond, I know there can be some spurious signals down in the weeds on one channel of the soundcard - which can either be overlooked, or the other channel used (some users find that noise aesthetically displeasing, even though it typically doesn't detract from the measurements they are making).
When I need to extend measurements beyond 96kHz, which I certainly do for Williamson amps and typically have to go out past 500kHz to assess feedback gain and phase margins, I use a separate signal generator and a scope and some voltmeters that have enhanced bandwidth, and so there is no opportunity for the soundcard to pollute such measurements. So I'm intrigued as to what type of measurement situation could be corrupted by a soundcard emitting noise well outside its nominal measurement bandwidth. Are you envisaging that the soundcard would be in use at the same time as when measurements were being made by other equipment (which could be corrupted)?
Ciao, Tim