Audiofiles with blue bubble do not work


I cannot use a large number of audio files for my presentation. They are having a blue bubble as icon instead of the itunes icon. I do not get why I cannot use the files. They are mp3 files, they are not protected, they reside on the same drive as the other ones I can use. There are albums, some of the tracks are usable, some not.

Would be great to get a hint.