Go get the most updated software from http://www.apple.com/itunes/
iTunes should ALWAYS reconise music files when you drop them onto it. It may be that you have a movie file or something that's not an audio file but is pretending to be one (Or it may be in an unsupported format). You could also delete the iTunes preferences. It would be nice to know exactly what version of OS X you have (10, 10.1, 10.0.3, 10.1.2, ect...) E-Mail me at kbotc@mac.com if you need more help.