Carrie wants to know this:
When you hear footsteps, do you hear "them" (footsteps) or "it" (the sound of footsteps)? Example: He heard them too; footsteps.)
Well, footsteps is plural, so you would use them; sound is singular, so you would use it:
He heard them too: footsteps. (I would say a colon, not a semicolon, is better there.)
He heard it too: the sound of footsteps.
Footsteps: He heard them too.
The sound of footsteps: He heard it too.