Did you fit a new radio and get it coded by Volvo? If not, it won’t work.
The Volvo anti-theft security system knows the serial numbers of the radio and amplifier in your car. It prevents units with different serial numbers from working, unless the serial numbers are recorded. This effectively prevents using units from other vehicles.
Stop replacing parts blindly, and start diagnosing.
You need to connect to VIDA and read the fault codes, and perform diagnostic tests. The problem may be very simple, or may not be. At any rate, it will stop you throwing money away on parts you do not need...
I just swapped out my High Performance amp for a used Premium amp. Turned the key to accessory and the stereo didn't turn on. Was concerned thinking I got a dud amp. Put my old one back in, same thing, no stereo power up. So I started checking all the relevant fuses. They were all fine. Then I noticed Low Battery warning on dash. Threw the charger on and all was good again. So yes, low voltage reading will disable your stereo - unsure if that was affecting OP?