AC, of course. Speakers don't work on DC. Its the alternating of the current in the coils (dynamic speakers) that makes the speaker move back and forth (the principle is different on electostatic spekers, but still depends on the alternating of the voltage). If you attach a battery (9v), which is a source of DC, to the speaker it will move to one position, front or back, depending on its polarity.
Now do you understand why it is nonsense to talk of negative and positive conductors and using different types of materials for these supposedly different conductors?
Salut,