Would a sledgehammer be okay to break up limestone?
Wear safety glasses and earplugs. I'm quite serious.

As commented, depends what you are breaking up, and how much, and to what extent speed is a factor.

A sledge will break rocks. A jackhammer will typically break rocks faster. An electric jackhammer might be the happy medium for "an enclosed environment" if there's a lot to be done, and electricity, without getting into the noise and hoses of an air-powered jackhammer. 

Then again, you might prefer to drill the rock full of holes (limestone drills pretty easily) to make breaking it up faster/more controllable (drilled holes plus feathers and wedges can let your sledge work more effectively at splitting rock in two rather than chipping off the edges.)

Again depending what you are breaking up, a diamond saw (wet are rentable, but you need to buy the blades - dry you can just use a cheap or unloved circular saw, and add effective dust filtration to your personal protective gear) to cut slots may be beneficial in making it easier to break up.