Terminal would be problematic, I think. Hiding/showing the extension is recorded in the Spotlight index for each file and I do not think that there is a way to manipulate that safely.
What you could do is using a Spotlight search in Finder and restrict results to MP3 files by typing ‘kind:mp3’ (without quotes). Then you select all files when the search is done (command–A), then open an inspector window with command–option–I, then check the box for hiding the extension. This will apply it to all files.