Korean scientists have just published their research on a stretchable speaker (that can also be used as a microphone) for use in wearables.stretchable.jpg
The transducer that they call a "Stretchable Acoustic Device" (SAD) uses a liquid metal coil, fabricated by injecting galinstan (an alloy of gallim, indium and tin) into an elastic form with a powerful neodymium magnet.
The speaker/mic could be sewed into clothing or applied to backpacks, hats, gloves, flexible phones, etc. In addition to traditional acoustic signal use, it could also be used for sensing biological signals (say a heartbeat or muscle contractions) or in hearing aid systems.
Here’s a link to the research paper and a video that shows the speaker/mic in action.