Keycaps generally don't change the volume, just the tone. Seeing as the WASD PBT doubleshots look a little thicker than their stock OEM ABS caps, they may sound a little "better". Thicker keycaps with more mass will tend to dampen more of the high pitch sounds and emphasise lower frequencies, whereas thinner, lighter caps will not dampen the high frequencies and tend to sound "cheaper" and more plasticky.
The material does make a difference, but not as much as the overall mass. POM tends to dampen the most, then ABS, then PBT, due to the "brittleness" vs softness of the material. However, the densities are in a different order, POM is densest followed by PBT and then ABS.
Anyhow, orings will do a
LOT more to dampen the sound than different material keycaps. If you still want to dampen the high frequencies, get heavy keycaps. The heaviest I've tried (and they're also my favourites) are Signature Plastics' SA profile (around 2g per cap).
I highly recommend using nice thick Cherry profile caps like those made by GMK (such as these for instance:
https://www.massdrop.com/buy/originative-gmk-dolch or these:
https://geekhack.org/index.php?topic=74079.0) with thin orings for using with MX Clears. They're a bit lighter than SA profile caps, but they have a consistent underside shape and so work well with orings. They don't leave much space between the bottom structure of the cap and the top of the switch when you press them all the way down, though, so they require thin orings if you don't want to change the travel distance much, but any orings will work.
