H.A.
The baby will be used to noises it hears while in the womb. So if your toddler screams, plays, runs around... if you have the tv on, vacuum, dog barks... none of those things will generally startle or upset the baby.
Don't try to keep a perfectly quiet house when the baby is born either. If you keep noises going when they are a newborn and don't tip toe around them - they will become used to the noise.
I play music for my kids when they sleep, I find that really helps give some soft constant noise and keeps unfamiliar noises from waking them. I have three kids and I have one that wakes easily, and two that sleep .. and could sleep through a train. I tried too hard on my second child to keep her away from noises, and she wakes if you even walk in and check on her.