Can 'would be able to' replace 'would have been able to' in the passage?

Question

I think it can't, but I can't explain why. could have heard can replace would have been able to hear, right? Thank you, in advance.

A sleeping mother has the ability to identify the particular cry of her own baby. This is one of the bonding factors that has been forgotten because of the way in which we live today. Typically, there is now only one newborn baby in any family house or apartment, so there is no way to test this ability. In an ancient tribe, however, living in small huts in a tiny village settlement, a mother would have been able to hear any of the babies crying in the night. If she woke up every time one of them screamed for food, she might get no sleep at all. During the course of evolution she became programmed to awake only at the sound of her own particular baby. This sensitivity is still there to this day, even though it is seldom used.

Answer 1

I extracted the relevant sentence for easier reading.

Original

In an ancient tribe, however, living in small huts in a tiny village settlement, a mother would have been able to hear any of the babies crying in the night.

Modified

In an ancient tribe, however, living in small huts in a tiny village settlement, a mother 'could have heard' any of the babies crying in the night.

The original example means a mother would hear any forthcoming cries.

The modified one means a mother possibly heard cries. Whether she did is unknown.

Answer 2

The title of your question asks about the difference between “would have been able to hear,” and “would be able to hear.” The former sounds more correct to me in formal written English, since a hypothetical scenario should be in the past subjunctive. The latter is acceptable too, though, especially in spoken English. Very few people use the subjunctive consistently.

In the body of your question, though, you ask about replacing “would have been able to hear,” with “could have heard.” This is grammatically correct, but as Seowjooheng Singapore points out, slightly ambiguous. “Could have heard” can mean either “was able to have heard,” or “may have heard.” In this context, though, I don’t think could have would be confusing.

I might also write, “... a mother might have heard any of the babies crying in the night,” to describe that hypothetical scenario. This is as short as any of the other examples, and less ambiguous than could have. (Many people do, admittedly, use may and might interchangeably.)