You'll need a pair of 120 Ohm resistors. Metal film or carbon film resistors will work well here, and 1/4 Watt or 1/2 Watt are adequate.
On the headphone jack, remove the red wire from the top outside contact of the headphone jack. Reattach this wire to the top inside contact of the headphone jack. Attach and solder a 120 Ohm resistor between the top inside and top outside headphone jack contacts.
Repeat these steps with the next level down, moving the red wire attached to the outside contact to the inside contact, then attach and solder a 120 Ohm resistor between each of these contacts.
-PB