Repair company caused a problem to my phone that was not there, how should I handle this?

I took my Iphone 5 to a repair shop to get fixed. My phone worked perfectly, except that it had static in it when I plugged in using an aux cord in the car stereo. I would hear a lot of static. It was like this for about 3/4 months. I had some money to get it repaired, so I decided to take it to a repair shop. They look at it for a couple of hours, and when I came back they told me they could not fix the problem. I got halfway home, only to find out that my phone had no sound. So, I took it back in and told them, I needed them to fix the problem they created. They said, okay leave it with us, and maybe we will get to the problem with the sound. Well, something said call them before I go. So, I did. I called the company, and the guy said, we tried taking it apart rebooting it, and doing something else to it. It worked fine, but then it the whole thing just started to fail. He said, the managers are not here, but when you come back, we will fix everything it is on our end. Well, I called back to find out when the managers would be in the store, and all of a sudden his tune changed to we experienced problems with the phone when we originally started fixing it! I was like no, that is not what you said. You did not make me aware there were any problems with the phone when I picked it up. You did not even tell me that you all tried to get the sound back on. You just gave me the phone back, and said nothing, like the problem you brought it in for we just could not fix. What can I do

Added (1). I just needed to give clarity: My phone had static in it three or four months before I took it to the repair shop. I took it to the repair shop, because I had money to fix it. I could hear my calls. I could here music when I played it. The only problem occurred when I plugged it up to the aux cord in my car to play music from my phone to the stereo speakers. I wanted to get it fixed. I left it with the repair guy. He said, he could not fix the static. He did not know what the problem was.

You actually expect us to believe you used a CORD, instead of Bluetooth to play music from your iPhone over the car speakers?
I don't believe you HAVE a sound system in your car that ALLOWS that. I also don't buy that the repair shop would do ANYTHING to the phone without insisting you replace the cord instead.
By the way, your story would be more believable is you said SOMETHING about the problem they allegedly caused.