T.R.
I am sure you are getting a wide array of advice, so I'll just toss my 2 cents in. lol
I am in the same boat as you, i have a 5 month old son and 3 1/2 yr old son. To keep him from waking his brother i have done a couple of things. First I make sure the room Mason (my baby) sleeps in has a constant noise. I use a fan.
Also I give him a place to make as much noise as he wants. He knows in his room he can yell, scream, jump around, whatever. If your lil ones room is too close to the baby's make it another room. I sometimes go in there with him and we yell and laugh and jump around. Discipline is sometimes not the answer with this age. They are pushing boundaries, but they also need to know we understand their needs too. Now I find that he does not need to make noise so much. He will tell me shhhh mommy Mason is sleeping. I try to put myself at his level, and understand what he is thinking. Hard sometimes with a three yr old!!
In our situation our lil guy was used to being alone, he was used to being able to do whatever he wanted. Now he is sharing his time and space with another child and I know it has to be hard. We have no family in here so it is just us, I was his playmate and his bestfriend. I try to make sure I am still that, so when I have to spend extra time witha cranky baby I try to spend extra time with him.
One more thing. We started what we call the goodboy jar. I have two canisters, one is bad boy jar, one is the good boy jar. We have these colorful craft ponpoms and when he is a good boy he gets to put some in the good boy jar. When he is bad they go back to the bad boyjar. When the goodboy jar fills up he gets a prize. At first it took a while to understand it, but now he knows what the consequences are if he looses a good boy ball. It has also helped me to remember to acknowledge even the smallest of things he does well.
Hope this helps some.
T.