We cannot fulfill our obligation to hear the reading of the Scroll of Esther (or any other mitzvah that requires hearing words or sounds, such as of a shofar) by hearing it through a loud-speaker, radio, television, telephone, Ipod or the like. These devices all change the human voice to an electric current or electro-magnetic waves, transmit it, and then reconstruct it back into voice. As far as Jewish Law is concerned, it is required to hear the Megillah read by a human voice, while in these processes, the human voice has been essentially changed and therefore is being heard only indirectly.

...the voice of a person is heard through mechanical vibrations formed in the air...

According to current scientific understanding, the voice of a person is heard through mechanical vibrations formed in the air, which may also seem indirect. The energy of the sound waves travels through the air, vibrating the particles until the vibration reaches the ear. But air is a tangible substance according to Jewish Law; particles of matter are actually vibrating. The movement of electromagnetic waves or electric current, however, doesn't involve anything tangible.

If, however, the listener hears through a device that mechanically amplifies the sound without transforming it into something else first, he does fulfill his obligation.

It is acceptable, however, to respond Amen to a blessing heard over a live broadcast (even though no mitzvah is fulfilled through this hearing). In this case it is not hearing a voice that is primary, but responding Amen at the moment the blessing is concluded. We know this from the synagogue in Alexandria described in the Talmud that was so big, signal flags had to be waved for people in the rear sections to know when to say Amen.

But why would anyone want to hear Megillat Esther over the radio or downloaded to an MP3 device? Go for a live reading. It is a big mitzvah. Also, the atmosphere can be quite inspiring, as well as exciting and entertaining. Doing so will also make it easier to fulfill the many other pleasurable mitzvahs of the day — Happy Purim!