Recently, I came across a telephone amplifier that I made many years ago. It's just a small transistor amplifier and speaker in a plastic box. There is a jack socket to plug in lead with a coil with a suction cup on the end of it. The idea was to stick the coil on the telephone so that it would pick up the magnetic field and amplify this.

So, by placing the pickup coil in front of say a tweeter, the magnetic field will be picked up and amplified, this can be heard through speaker in the telephone amplifier. I will, of course, be a limited frequency range, limited by the crossover. If placed in front of a mid speaker, similar result, biut of course mid range sound.

This is the sort of pickup coil used: http://www.ebay.co.uk/itm/A171-TELEP...-/111928287671