a bit of aproximatology:
let's assume a starting speed of 30 m/s, and a stone circumference of 10 cm, or 0.1m
IF the stone is simply rolling out of the pouch, tha would give us 300 rotations/s, or 18000 rpm
Obviously, there's a important "slipping factor" witch would severly reduce this incredible number . I guess there's an "angle factor" too, to reflect the fact that the stone may not get out in a perfect way.
Those petty considerations leads us to no answer, but show that 2 or 3000 are perfectly realistic.
As for the sound effect, consider that the audible frequences (for a human) are between 20 and 20000 Hz. that's au very wide fork, but perfectly suited for a sling stone...
Well... feel's like I'm going nowhere...