2013 Elantra ipod cable

I refused to buy the $35 dollar cable from the dealer, so I bought an iPod cable off eBay for $4.40. However, when I plug in my iPhone 4S I get the music to show up on my display and can control it from my steering wheel controls, but no sound. I have the option to put the audio source as the dock connection or the Hyundai and neither work. Who knows what the problem is? Thanks in advance.