At first this dB setting is not "normal" volume. It determines the distance at which you're still able to hear a CfgSound on an island.
The only thing you can do with dB for the "real" volume setting is getting it lower with a very very negative value, but I think that's nothing more than a sideeffect.
As CfgRadio is always played at 0 distance to the player, of course the dB setting has no effect.
To change the real volume just amplify your soundfile with a sound editing software.
Also the FAQ is wrong.
I know I've seen percentage values used in some custom addons, but maybe they were based on this FAQ, and I don't think they are right. (I don't think there's a difference between defining sounds in config.cpp's or description.ext's)
In my missions I always have to use big dB values, either positive or negative (i.e. -15, 45, 78, -120). You'll will notice it's the only way adding dynamic range to CfgSounds.
Percentage values will be interpreted as a positive dB setting between 0 to 1, and that doesn't make any noticeable dynamic range difference, from leaving the dB setting empty.