I re-use certain audio files among many of my Audacity projects. To make them easier to find and open, how about a File > Favorite Files > [pop-out submenu listing my favorite files]? This could work just like the File > Recent Files > [pop-out submenu listing the 10 most recent files].
Implementation would require some way to populate the submenu; that could be manual, where the user tells Audacity which files are favorites (e.g. right-click them on the Recent Files submenu) or automatic (with Audacity ‘remembering’ which files are opened frequently). Or, some combination.