Problems Burning an Audio CD- PLEASE HELP!

I am using the latest version of Nero Express and I haven’t had any trouble until now. The thing that I have noticed is that when I look at the MP3 file itself the writing is in blue and not in the stereotypical black/grey. What is the difference? I think this is why I am unable to write files to a CD. It locks up on me and says that the files were unable to be attached.

Thanks in Adavance!,

                         Vertigo21