I have the same issue, I've been using an s2 up until getting the s5 this week, same thing, i play my music thru the car stereo and can hear calls thru car speakers and the mic on the phone picks up my voice(S2) but on the S5 everything is the same except mic is dead with the aux cable in. I've had a look thru the settings but cant find anything obvious...did you get your issue sorted?

 

Do you have the media volume on the phone turned up? That's all I can think of. Mary

 

my aux completely stopped working including my headphone jack i replaced it with a new model and its working fine now they seem to break very easily as it happend for no reason.