Y'know, when I first bought the car - my stereo was able to determine the difference between Bluetooth Stereo playback, and a phone call. But lately either the BT on my iPhone 6 is failing or the BT module in the stereo is crapping out, because the phone will all of a sudden disconnect - then the Elantra's BT system assumes its a phone call, and plays the music on the stereo in crappy static-like sound, much like you're describing.
I wonder if YOUR system just wasn't able to differentiate between BT Phone and BT Stereo playback, and its stuck in BT Phone call playback...?
I've resorted to plugging my phone in to play music, but I hate it. I would rather the BT worked properly... :/