The grammatical structure of "I heard people sing"

The structure "I heard people sing" is correct in grammar in common sense. But why is it correct? The common structure "S(ubject)V(erb)O(bject) tells us that we should have used "I heard people singing" (both poeple and singing serve as the object), or should have used "I heard that people sang" (the clause started with "that" serves as the object), but "I heard people sing"? What role does the verb "sing" play here? Is it actually meant "I heard people to sing?" With "to" it looks and sounds awkward anyways.