The envelelope frequency for the signal is on the same frequency range as human voices are, the vocal chord nerve frequencies that is.
There are sound devices that work on this exact premise by picking up the electrical signals being sent from the brain to through the nerves to the vocal chords, maybe not exactly an artificial larynx (which works via pneumatic vibrations) but something quite similar to it does indeed exist.
Anyhow, assuming that people's nerves can pick up on the AM radio signals broadcasted through the air, then it's possible that the signals might actually be running back up those nerves and influencing people's brains, somewhat like was shown on the History Channel episode of MIND control.
Sources:
Pictures 1,2,3: Electronics - The Easy Way 4th
Picture 4: vocal chord nerve frequencies
Picture 5: Artificial Larynx
Be sure to check out the History Channel video:
That's Impossible - MIND control.
