why the volume goes down on my radio?

It's an option that some vehicles add as part of the drivers comfort. Most older cars didn't have it. It's designed to keep the volume of the radio at the same audible level throughout the RPM range. As you accelerate, your engine works harder creating more engine noise. As well as the wind and environmental noise getting louder at higher speeds. If you have the volume at a comfortable level at 20mph, you will probably not be able to hear it well at 65 mph and will turn it up to hear it adequately, this feature just does it for you. If you have it blasting all the time, you probably won't notice it getting louder. It won't blow the speakers. It will have a cut off point that it recognizes that it won't go past. If you have it louder than that cut off point, it will not go louder on it's own. As far as the volume reducing at lower speeds, it's doing the same thing as when you accelerate. It's just keeping the volume the same dependent on your speed. As far as being able to turn it off, i'm not aware that you can do that. But like the previous poster said, check your manual.

it's a feature!

Speed controlled volume I think it's called.

Check out your owner's manual if you don't like it, it can probably be shut off.