IPhone – How to tell if a 30-pin USB cable for an iPhone is genuine

That's a third-party cable, not an official Apple cable, so what's probably happening is that for some reason that cable is bridging the 2nd and 11th pins, telling the device to send the audio out the 3rd and 4th pins:

From http://pinouts.ru/PortableDevices/ipod_pinout.shtml:

2 GND Audio & Video ground (-), internally connected with Pin 1 on iPod motherboard

3 Right Line Out - R (+) (Audio output, right channel)

4 Left Line Out - L(+) (Audio output, left channel)

11 AUDIO_SW If connected to GND the iPhone sends audio signals through pin 3-4, otherwise it uses onboard speaker.

If it works through the regular 30-pin to USB cable, it should work with the Lightning to USB cable as well.

This type of connection is through an all digital interface and I am not aware of any changes to it, when the lightning connector was introduced.

It's a different story, when you have a Y-Cable for your car. This would have a dock-connector on one side and a USB and Headphone-jack on the other.

In this case you will need a Lightning to 30-pin adapter. This would be an analog connection and the adapter would do the conversion from a digital into an analog signal.