[Nyororin]

For your first question, I am going to assume you mean にばんどおり. I`m sure you will agree that in English there is a difference if you were to say "Second Street" and... "the second street". One is a name, the other is referring to the second in a series. The same deal in Japanese. You can say either but there will be a difference in meaning.

When it comes to your second question there is a fairly simple answer. すこし refers to amount. ちいさい refers to size.
You want the sound of the radio to decrease in size, not in number, so you would use ちいさく.
You can say ラジオの音をもう少しちいさくしてください - You`d be asking that they reduce the size (via ちいさく) of the sound by a bit more (via すこし).