Be honest. A lot of things have to change when you have kids. A lot of things don't. But they're expecting to be able to maintain the same lifestyle while their friends make the sacrifices. It doesn't work that way. "I love you guys, but get-togethers aren't that much fun when we have to whisper all the time." "Can you come out this weekend, and Mark the next?"
White noise. I learned to love it very early on. A box fan set on medium will drown out almost anything. They also make white noise machines that cost very little. A towel crammed up against the crack under the door really helps, too, if you have one of those old, echo-y, hardwood floor houses with big spaces under the doors. Also, if you train your kid to sleep through voices in the other room, he will be very grateful as an adult.
I have a girls' night at my apartment every couple of months. Fifteen ladies in our little living room are not exactly quiet. We're like a herd of hyenas, actually. My kids sleep straight through it. The white noise drowns out a lot of it, and the little bit that you can hear in their room, they sleep through. As long as no one starts yelling in the hallway (they don't), everyone's fine.
When it comes to going out, my husband and I take turns. A babysitter every weekend would really add up. Hey, it's less fun than going out together every weekend like we used to, but it's better than whining at people who have the audacity to not bend to our new lifestyle.