OH and another great great great tip I learned is that you should NOT only shower your pets with attention when not holding the baby. If you're nursing or feeding baby and shoo them away they will begin to associate baby as an "enemy" coming between you and them. Instead let them snuggle near you while you nurse/feed baby and talk to them once in awhile. If you only give them attention when baby is not around that only reinforces the idea that baby is bad. They have to learn to get along. ;-)
I agree baby will be used to the noises your pups make and may actually be soothed or comforted with the familiar sounds.
Just some very obvious tips that you probably already realize:
-NEVER leave pets alone with baby, even if you just sit baby in a bouncer to walk into the kitchen for something. I always shut our baby girl's bedroom door when I put her down at night after I make sure the cats are not in there and my daughter is well over a year old now.
-Watch the chihuahuas as they are as small as cats and may want to nibble or lay on baby
I think it is just common sense and bringing something home from the hospital or letting the pets sniff the baby when you arrive home helps them to acclimate to their new buddy.
I agree with Joy about not having to give pets away unless they are violent or somehow they effect your baby in any way. Like allergies or sometimes cats have issues with smothering babies but you don't have to worry about cats I guess.
I think it's true that they sense it because my boarder collie followed me around obsessively when I was pregnant but it's odd because now he doesn't pay any attention to our baby. He'll come up and sniff every now and then but that's it. I thought he would have been annoying and be obsessed with the baby or jealous and he's not.
Something you can do is when you're in the hospital, get DH to bring home one of the baby's blankets for your dogs to smell and get the scent before bringing the baby home.
If the dogs bark a lot, I wouldn't worry about it bothering baby. I think they get used to hearing the barking in utero!
Dogs can usually sense you're pregnant and "with child" and I think they'll be more curious once the little bundle arrives or maybe not notice at all. Though judging by your mastiff's loving nature she will most likely become like a protective momma to the baby. I don't think people need to get rid of pets when a baby comes unless the pets become violent.