Making passive speakers louder
Headphones jacks aren't intended to drive speakers directly. Headphones differ from loudspeakers significantly in two ways:


headphones typically have substantially higher impedance, some hundreds of ohms, compared to the typical 8 ohms of a loudspeaker.
headphones, being right next to your ears, need substantially less power to achieve a suitable volume.


To solve this, you need an amplifier somewhere, one intended to drive loudspeakers. No way around it. An iPod (and likely any other device with only a headphone jack) can simply not output enough power to drive loudspeakers without help from an amplifier designed for that purpose.