In my experience, tuning can make a huge difference. Nock fit is often overlooked, as is silencer placement. A clean release is quieter, as is a heavier arrow.
With strings...generally how the string is made makes more difference than the material, although some materials can be quieter on some bows. I've had customers tell me that a Dynaflight '97 string made their recurve as quiet as any longbow they had shot. I got regular comments about how quiet my (deflex/reflex longbow) was with a Dynaflight '97 string on it. Used properly, it's not a noisy material at all. Often people confuse pitch and volume--we tend to think a higher pitch is "louder" when it isn't. If I remember correctly, a low pitch travels further.
You have 3 basic types of material. Polyester (B-50 and B-55), HMPE (Dynaflight '97, 8125, 8190 Universal, 8190F, Fury, etc.) and HMPE blends (450+, 452X, BCY-X).
The most recognized brand name of HMPE in the string material market is Dyneema, which comes in different grades. The most widely used is SK75 (Dynaflight '97, 8125G, 450+, 452X) and SK90 (8190 Universal, 8190F, BCY-X).
In my experience, the absolute quietest materials are the Dyneema/Vectran blends, but not by a huge margin.
450+ has a big percentage of Vectran in it (30%), which is a brittle material so it fuzzes up faster and isn't as durable.
My personal favorite is BCY-X which is only 17% Vectran with the highest grade Dyneema currently available. 18-20 strands works great. Quiet, stable, consistent, durable.
Again though, generally speaking and when using the better materials there's not usually a big difference in noise.
For me, tuning is probably the #1 factor. That includes silencer placement. Then an arrow that weighs at least 8 or 9 gpp, and a clean release.
Chad