Radio volume keeps going down

Hello, does this car have the Remote-Control radio system? Unfortunately, your mechanic may be correct, the speakers all come directly out of the amp. Unless some other input is turning the volume down on you. Do you have radio controls on the steering wheel? In the diagrams below, the first one is the Amp Fuse, and the second one is the speaker wiring coming from the Amp. I will post any additional information as I come by it.
Does this have a DVD system as well? If you can turn off any voice control or remote systems that may help. This system is pretty complex. The radio and amp are on the CAN bus network. That means they could be receiving signals from any module on that network.
The third diagram is the network it's on. I would think that if it was an Amp failure, it wouldn't just turn down the sound on you, I would think you wouldn't have any sound at all.
I don't see any technical bulletins on this issue. But the possibility of something interfering with the volume controls is possible.
The only tests there are for a "No Response" condition from the Amp. Which is only checking the 2 power feeds to the Amp.

The diagnostics page for the Radio/Amp system states to start by using a Scan tool to check the communications for the Radio circuit, so this is probably not just a blown fuse issue. It also states that a cell phone can interfere with the wireless system.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

I was looking through some information last night, do you use the Bluetooth option to talk on your phone while you're driving? Where you would hear the person, you are talking to through the speaker system on the vehicle. Or possibly the GPS system threw the Bluetooth and receive directions through the radio speakers? There are a couple Ground locations you can check. Under the passenger side lower kick panel, this is where the Amp is located also. It's worth checking that there's no loose Grounds or connections at the Amp itself.