At the moment there is no way for you as a player/user to change the volume of the radio jukebox.
I will look into increasing the volume of the vehicle jukebox, but be aware there will be situations where the audio coming from things around the vehicle will drown out the audio coming from the jukebox, e.g. When in a helicopter or aircraft. These are unfortunately issues that we cannot fix due to the fact that all the audio is playing on the same 'effects track' in layman's terms.