I think it's common courtesy and respect to lower your volume level when anyone is sleeping, regardless of age. Most kids will sleep through background noise, but sudden loud noises will usually wake anyone. An adult could probably just fall back to sleep, but babies and kids don't rebound from startle quite so easily.

I was surprised that my youngest slept through the festivities at my parents house on Christmas day because their house is long and thin so there is no room "out of the way" for her to sleep in. All the rooms are right off the corridor that people go back and forth down. Even my other two kids were going up and down it with rolling toys (on a tile floor *sigh*). I didn't warn everyone she was sleeping though and I think my family would have made an effort to be quieter if I had asked