I have a project in 1.5.6 where only one file is used in one event but there a many files in the audio bin. I discovered the ##unused trick in the manual, but it doesn’t find any files. I’m on Mac OS X.
The manual is currently out of date; The search term you need to use is “#!unused”, rather than “##unused”.
Note that files removed from the Audio Asset window won’t actually be deleted from the project folder until the next time you save the project, so don’t be alarmed if your project size doesn’t drop right away.
Oh, and files that aren’t used in events don’t get included in built banks, so the size of your built project won’t be affected.