Converting heat energy directly into sound using tiny electrical conductors is a 100-year-old idea for an alternative to the mechanical voice coil wire + moving diaphragm design of traditional speakers, but new research recently submitted to Applied Physics Letters demonstrates a new, actually feasible approach to making these speakers-on-a-chip. Still way too quiet and underpowered for use as a loudspeaker, but might have some novel applications in the near future as research progresses.
I like the name given to the 100 year old invention, though: the thermophone.