Long have I yearned for an Ethereal Ball of String.
Throw yourself back in time, throw off the shackles of sensibility, see the world with a childlike sense of wonder, and you might convince yourself that a ball of string really is endless. Deep inside the knotted mass of threads, the entanglement transcends Euclidean geometry and connects to another dimension.
The Ethereal Ball of String could be unwound indefinitely – if not for the knotted mass being impossible to untangle. But there's still magic inside.
The end of the string is not a loose, fraying thread. The free end of the string has a tin can!
A tin can connected to a ball of string would, in and of itself, be a fairly useless device. But the ball of string is special, the twisted thread coiling up and connecting us to that other dimension. A wormhole. A connection to the ethereal plane.
Who can say what tortuous path our string takes through the ether? But somewhere, some potentially distant location in the real world, there is another ball of string. And that ball of string is of the very same thread as ours.
The two balls of string are entangled. The two tin cans are connected, the strings as one through the ether.
If battery powered, the string tension would also function as a power switch. Ideally, we would power it from the user somehow, but the energy of spoken words is far less than what's needed to run a wifi microcontroller. It's probable an inductive charger is the best option.
The transducer may be difficult to design. It has to support quite a bit of string tension for the tin can mechanism to work as a speaker, but at the same time needs to be free enough to pick up the vibrations. Probably some kind of spring-loaded mechanism with a coil to pick up and create motion, the spring tension keeping it from bottoming-out. Alternatively a piezoelectric crystal might be fine.
From a technical point of view we would need some serious echo-cancellation. Using the same transducer for both sending and receiving will inevitably cause problems. It may be easiest to simply threshold the input and make the communication half-duplex.
There was a trend at one point for sticking bluetooth headsets into unassuming objects. I've been guilty of it myself. But one of the funniest ideas (which I couldn't bring myself to copy directly) was to take an old fashioned handset from a landline telephone. The coiled cable just dangles to a frayed end, and it looks like you're talking into a disconnected handset – but the call really works.
In the same vein I'd like to have a morse code key that taps out on some entangled key elsewhere without any visible connections. And perhaps, straying into the territory of the Jack Sparrow Compass, a pocketwatch that doesn't tell the time. Moving the hands would move those on the entangled device. A codebook could be worked out so that certain hand positions convey surreptitious messages.