なるほど！ Dankjewel:)

On second thought, my interpretation may be wrong:( I don't know the whole lyrics of the song, so understanding the meaning of this part alone is very difficult. 「口無しkuchinashi」 may be a specific character and the character (who doesn't say a word or cannot speak at all) actually sings a song. Show the whole lyrics to your Japanese friend and ask him or her about the meaning of 「口無し」.

Oh, 「口無し」? Maybe that Japanese thought 「口無し」 is the name of a character like 「かおなしkaonashi」 in "Spirited Away" (「千と千尋の神隠し」). Actually 「口無し(kuchinashi)」 is not the name of a character; it is the name of a tree ("gardenia"). We call it 「口無し(literally meaning "without a mouth")」 because the fruit of this tree does not open even when it ripens. So "kuchinashi" has double meaning in Japanese: a person who does not say a single word and gardenia. So, 「口無しは唄う」 is a bit strange and contradictory sentence, but sounds very mysterious. That is, "A tree that is thought to be unable to sing because it has no mouth sings a song" (=> Japanese people would think "Oh, it's impossible, but I want to hear the song if it really sings."). This is my personal opinion, of course;)

Hmm, it's a bit tough question for me 'cause I don't know in what situation you want to use these words, but I'd use these two words like this: 「今、一文無しです。」(I'm broke now.) 「罪無き子供」(an innocent child) I hope these example sentences will help you:)