Suppose that instead of stringing string between two cups to make a telephone, you string the thin copper wire I talked about in Copper Segelín and produce the sound by heating the wire with joule heating by passing varying amounts of current through it, thus making the wire expand. How much current do you need, and what does the frequency response look like?
Well, you probably need to stick to a few hundred mA to avoid melting the thin wire, as calculated there.