The RPM/V on a motor depends mainly on how it's wound (aside from friction of course).
The DC motors used in R/C servos are quite small and of comparatively low RPM.
The SPL of a motor is usually inverse proportional with the quality and the quality is proportional with the price (loosely speaking), which has to do with the type and quality of the gears and bearings, the engineering tolerances and so on.
You can get a DC motor of that approximate size for under $2 and you can pay hundreds of $$ (if not more) for a precision engineered motor of the same RPM and torque.
One example is the Dremel which is slightly better than its clones, but hopelessly inferior to the professional equivalents that goes 50,000 RPM without you even feeling it's on (unless you slice a pinkie