My apartment was built with concrete floors that do a good job of insulating the sound. All the same, the neighbors below can still hear things sometimes, especially if my fiance is doing plyo with me -- she doens't land too softly

They're pretty cool though, and as long as we're not doing it in the middle of the night, they don't care. If you're in an apartment you're going to expect at least some noise from the neighbors. It's part of the apartment gig!
Just make sure your workout space isn't directly above any bedrooms, and try to workout during normal daytime hours. And as others have mentioned, really try to focus on landing softly. If you're too tired to land softly, then you're also too tired to use proper form and should probably take a breather anyways!