I would tend to think barking more or less might just be an aspect of the dog's personality. For example, a more neurotic dog would likely bark at every little noise. Hopefully, you're just lucky and got a dog with a personality that doesn't bark much. I see that as a good thing, mine barks a little if someone new comes to the house, but otherwise it's pretty rare for her to bark. Most people say chihuahuas are "yappy", but not mine. I don't think mine is lethargic either, she runs around the house and loves to play fetch and generally get exercise. Which isn't to say I don't recommend getting your dog checked by a vet just to be safe, a general check-up is certainly a good idea anyway, but hopefully nothing is wrong.
