Why is it called Bas Kol versus Ben Kol?
The Tosafos Yom Tov on the last mishna in Y'vamos (16:6) says that the diminishing volume on each register of an echo is tantamount to weakening. The reverberation is thus associated with the fairer sex.
He also distinguishes between the type of bas kol that is simply the echo of a (meanwhile departed) human's voice and the type of bas kol that is some form of prophecy. Since the the mishna is dealing with the former, the explanation of the terminology might also be limited to that type. Whether there would be any reverberation involved in the prophecy-type bas kol is unknown to me. Since that is the basis of the explanation of bas as opposed to ben, this could be significant in fully answering your question according to the Tosafos Yom Tov.