Every media item allows you to add your own tags. Look for the Tags field in the Info View. You can add multiple tags to any item which allows media items to belong to any tag collection. To see the audio or video tags collections choose Tags from the Media Lists selections on the left of the Media lists view. These are Nepomuk tags so they are available to the rest of the KDE desktop. This also means that if you add a tag in Dolphin, it'll show up in Bangarang.
