I own a Kia a Rondo 2012 and I can connect my iPhone to the sound system using Bluetooth. It does work great to take a phone call but I've been struggling to play my music on my car radio over the Bluetooth connection.
Sometimes it would work by activating Siri and then pressing Play right after cancelling Siri. It was a real disappointment as my iPhone was seeing the bluetooth connection but nothing was coming out of my car radio.
That is until tonight. I finally figured it out. The issue was not my phone but my car radio. In the sound settings, there is an option to turn "On" the "BT MP3". For some reasons, it was turned off.
After activating the setting, I just start my music on my iPhone and it works as it should have been from the start.
If you are facing the same issue where you can't play your music on your car radio over a Bluetooth connection, just have a look in your car audio settings (Balance, Bass, Treble). You may have to enable Bluetooth MP3.