I suspect the galaxy is detecting open circuit. It needs some impedance to activate the audio.
The aux connection wiring is simple, but complex. It has from memory a 200kohm resistor across the head unit inputs to trigger the AUX function in the head unit. But to protect this "AUX trigger" resistance level the AUX socket audio signals are then capacitively coupled to the head unit. If this was not done, any device connected could reduce the 200kohm thus disabling the AUX function. This means that any device connected to AUX does not "see" the expected 100kohm impedance. This is quite ok for device that do not have intelligent output switching. The galaxy does have automatic switching hence a problem. You could fix by opening the socket and connecting a 100kohm resistor from each input to ground. Or maybe there is a hack available for the galaxy to override the automatic headphone detection.
X3 3.0d 2006 (MY07)