Multimedia sound problem (volume doesn't display)

If it is a JBL system you might be able to make your Prius ready and hold down the power on/off button for 5 seconds or so. This will reboot the unit. That should happen during a regular power on/off of the vehicle too, so that might not help. Only other thing I can think of is something getting down inside the headphone jack, which on my vehicle is integral with the USB auxiliary socket, and obstructing the speaker or headphone option of that socket. But the multimedia screen would still show volume changes. Did the "heavy rain" get inside the vehicle?

 

Clarification needed, the power button you hold down for 5 seconds or so is the infotainment system power button.

 

Do what you can to dry the vehicle out. That may be the cause if water gets into the MUX signaling harness. Have you checked any fuses yet? Not just visually but with a continuity tester.

 

I don't think we have established yet if your entertainment system is original equipment or aftermarket. To my knowledge the OEM is JBL. Anything else will have different integration schemes into the existing vehicle signal and electrical system. I know of two fuse blocks in the engine compartment and there may be some more in the interior under the dashboard. I don't think a water intrusion short would be enough to blow a fuse, but it sure could interrupt a low voltage signal path. Did you dry out the interior yet? If it were my vehicle I would likely have removed just about everything from the interior down to bare floor, traced out the wiring harness, and cycled all the connector plugs and sockets. Big job.