I see many models with db ratings of 35-36 in dooyoo.de. Bosch and AEG.
Nice. Too bad they're not available.
1. This fridge apparantely has a fan that blows to the freezer department (I assume for defrosting). This makes a bit of noise too. Is there a way to disbable that (I know not!)?
Unplug and cover the wires. Probably lose some functionality, but doubt the fridge would cease to operate. Prepare to defrost manually every three months.
2. If a same brand has 3 different size of fridges in the same model line... let's say 200 lt, 250 lt, 300 lt... Would all of these fridges be using the same compressor? Or would the smaller model might be using a different compressor with less noise/less power?
Unknown. Smaller capacities take less time to cool, but same applies to warming up. Big with solid elements will need less cooling runs than small if the basic construction is the same. See question #5.
3. Would the sound increase as the capacity of the fridge increases? I mean, would the noise of the compresor increase if its power increases?
Unit-dependent. I have experience with aggregates mostly, but you can never tell of the noise by size. Factors like insulation and installation count. The sound will be different for every compressor, usually the big ones have more of a rumble and the small ones have a chirp - just like with all machines.
Usually small, efficient unit in solid casing is the best bet.
4. The freezer compartment has a setting of 1-to-9 and the refrigerator compartment has min-normal-max. My freezer is almost always unused. Which setting would work best to keep the fridge quiet or to stop the compressor from engaging? Freezer:1 Ref:Normal?
Probably. It'll at least cut out freezer cooling sooner. Hard to say what the overall effect will be, use trial and error(and read the manual).
5. -- which one would be more advantageous soundwise: Keeping the fridge empty (air) or filling it up completely with mass (water)?
It would balance temperature changes, yes. The water will keep its temperature better than air that just gets out when you open the door. Never tried that myself though, never had to. Doubt the weight of the bottles would be enough to make a difference.
If you're going to use plastic bottles they might result in increased condensation.
I'd still advocate DIY to direct the noise away from sensitive areas, maybe add absorption materials to reduce echo. Just had a browse through some sites, not terribly expensive, especially if you can score some old studio materials. Happy hunting.