i am not too familiar with alpines, but haven't seen many others posting about them(this forum is primarily for installing computers in cars), so..
my pioneers used to have a aux input level adjustment to help make it louder. not sure if alpine has the same feature..
also, make sure that your devices headphone output is as loud as it will go..
to my limited knowledge, most aux input adapters for radios are mostly isolated, so i don't believe it should be a grounding issue, though i've been wrong before..
if possible, maybe try some other devices? both plugged into car power, and unplugged?
it might also be worthwhile to attempt to reslot the connector-- maybe it is 'just' barely connected, and is causing the problem.