Mine is half parsons and half JRT. She has a deep bark for her size that I wouldn't call a yap, but, she can be a noisy little sod.
Never had a small dog before her and so I trained her the same way as all the others I've had and she does stop when told. She does like to respond to other dogs and whines at me when I tell her to be quiet though!
She's very vocal though when playing/being fussed with a growly whiney type noise, grumbles a lot when stretching/yawning and when she gives herself a good scratchy (or smelly!) Roll.
But the Westie I had was far worse. Knew she shouldn't bark outside, and would be brought in when she did - it got to the point where she'd start bark and then turn herself around, and bring herself in without a word from me, and that was a yap!
Family members jrt is a yappy little sod but she's babied and untrained, and that's a true yap, pierces your brain!
I think pp is probably right, it's the way they're trained, had the jrt from a pup, the Westie was a rescue I got at 5.
Mine is definitely more vocal than other breeds naturally, but it is possible to train them to not be as bad. My neighbour says they never hear mine when I'm out, unless someone knocks the door and then they quieten down fairly quickly. Only problem I've had is she howled when left alone after the Westie died, but we had another dog that came shortly after and she doesn't any more.
Destroying balls and raiding anything food related left unattended for seconds is a different matter though!