Because there will be a current flow of whatever value there will also a electric and magnetic field produced. Regardless of the frequency you use.
But the range will not be far because a Speaker is constructed to hold his magnetic field inside of the unit to drive the Membrane of the Speaker.
So it is not a good device to transmit high frequencies or higher frequencies because the menbrane is too slow to follow these high frequencies.
Definition of high frequencies: > 20 kHz
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