Can you advise the model of the Sony headunit, this is likely to be one of two issues.
1. The mute feature has been enabled in the settings
2. There is a lose wire on the DIMM plug at the the back of the headunit.
I assume you have tried un-muting the radio by pressing the mute button on the steering wheel?
Once I hear back from you I can see what we can do prior to needing to get an engineer our to investigate the fault.
You would normally find the model number in the entertainment user guide \ manual which came with the car.
Assuming you have a stick on the steering column to control the phone and voice control, I can confirm there isnt a mute button on that, the mute button is only present on the controls which are built into the steering wheel.
The gen 4 radio has a mute button under the red end call button on the right hand side of the radio, since you cant find it, I believe you have a gen 3. I have however come across two things you can try which may fix the problem (if these fail) it would point towards a hardware failure, and need to go in to a dealer as you have to remove the glove box to get to the unit which is no easy job.