With sprites in the editor, I can choose to "replace sprite from source", refreshing the in-game sprite with the contents of the .png from which it came.
With audio, I can see that it remembers where the sound file came from originally (Sourcefilename), but it appears to me that in order to change the sound in-game, I'll need to delete the existing sound from the editor, then re-import it. Is this correct, or is there a way to "replace audio from source" that I'm missing?
I've wondered the same thing actually - my current method is to do what you've described, deleting and re-importing a file with the same name (which at least means you don't need to update any code); I haven't found any way to replace the file from within the editor.
Delete AudioCache folder from your project's directory.
But you are right, the option to do this from Editor is missing. Might be a good idea to add a feature request:
http://www.adventuregamestudio.co.uk/forums/index.php?project=5;area=issues
This was done already in https://github.com/adventuregamestudio/ags/pull/2074 and made into 3.6.1 release I think