This teshuva is Rav Moshe's original teshuva about using a microphone for Megilla. Unlike most of his other teshuvot, this one is undated, although it was obviously from before 1980 (when OC 4:126 was written), and judging by its content it seems to be somewhat early.
Rav Moshe advises not to use a microphone for Megilla, although he concedes that the issue is not clear. He discusses a bit of the science involved as to whether the sound produced by the mircophone is an actual voice or merely an echo. He waxes a bit philosophical in terms of how exactly we are to define "voice." At the end of the day, while he is not sure that someone who uses a microphone for Megilla is completely wrong, and while acknowledging that allowing it for Megilla would not create a rush to use it for other mitzvot, since many other relevant mitzvot such as Shofar and keriat HaTorah take place on days when microphones are forbidden anyway, Rav Moshe nevertheless rules against this innovation, if only in order to slow down the rush to embrace innovation for its own sake.