Transcribe was greyed out for me too, for an .aif file.
To transcribe I selected the file in the project window, went to the Clip menu>Audio options>Extract audio. Not sure what that does for a file that is already audio. This created a new file with a .mov suffix. With this new file I was able to choose Transcribe to text. Then the text will appear in the metadata tab, albeit not very accurate.
Shahane