I think you're considering two alternatives:
A) If they listen carefully, they might hear the woodpeckers.
B) If they listened carefully, they might hear the woodpeckers.
It's hard to imagine the situation in which anyone would mean to say the second choice. If they listened carefully (in the past), they might hear the woodpeckers (in the future)? Why would the fact that they listened in the past cause them to hear something in the future?
The problem is not grammar; both choices are grammatical. It just seems like choice A is the only one that would arise in common situations.