For a long while, Snap! has had options at the end of the file menu for picking from a collection of costumes and sounds:
These show a dialog pretty similar to the choose-costume/sound dialog in Scratch:
These are quite handy for grabbing quickly grabbing a costume/sound (although the sound library is a little lacking in quantity :)). So, it makes sense that they should be more prominent in the editor than being placed at the very bottom of a menu. So that's my suggestion - to have a new button in the Costumes and Sounds tabs, beside the existing paint and camera buttons, for opening up those dialogs.