Some examples of how to do that are in the Status Bar Codes thread.
Or just select the relevant thing from the menu in Preferences which contains all of the things you can put in the status bar.
Because nobody has written code to do so. 
Actually, I have just written code to do m4a but it's not ready for release yet.
I didn't realise Opus didn't do WAV duration, actually. I'll add that to the same plugin I've been working on.