Akustica is a US start-up company (not to be confused with the journal of the same name), that is developing MEMS that have combined on-chip microphones and electronics, and also speaker and electronics. This new technology allows the components to be miniaturised and produced extremely cheaply. They have developed microphones that are less than 1 mm^2, and a miniature speaker array that has 8 individually controllable speakers.
Read about the technology on the Akustica web site.