Okay. To also mention the result: Installation worked, alsa plugin worked and the filter does work. Nice, thanks again.

With extreme sounds (vacuum) in the background the voice gets a bit more distorted than ideal, but something like a keyboard gets filtered nicely to be less noisy. I assumed that's just how RNNoise behaves, I'm just mentioning it because of the sound quality discussion above. Maybe also to that: Just activating the alsa_rnnoise filter does not significantly lower recording quality, at least not that I can notice.