Printing "audo.duration" to a text object?

Ok this one should be simple. I want to print "audio.duration(Tag)" to a text object, but I can't find a way to make construct understand I want the duration of a given tag. "Audio.duration(Music)" for instance doesn't work? How do I write the string-line?

Edit: Or well, It's the "playbacktime" expression I want, but according to the doucmentation they should be working the same way.

Thanks