Unfortunately, the likely cause of this is the radio has failed. I have seen this a couple of times and a radio replacement has repaired it.
If the knob on the radio worked but the buttons on the steering wheel did not, then we could have another issue but since they both don't work then that means the radio is the issue.
If you want to confirm this, then we need a scan tool to monitor the CAN bus network to see if the radio is receiving the input to change the volume.
Here is a guide that talks about this network:
Please see the attached info on the radio replacement as I suspect this is what will fix the issue.
Let us know if you have other questions.
Images (Click to enlarge)
Saturday, March 19th, 2022 AT 6:19 AM