Edit Sound Externally opens the currently-selected sound clip in your preferred external editor.
It's only enabled if the current selection in the Story Tree is a sound clip that is already split. If it's a sound clip that is unified (not split), the Split And Edit command is enabled instead.
If you haven't saved the file since the sound was split, you will be prompted to do so. This ensures that the split file will exist.
The split file is then loaded in the external editor. Once it's open, you can edit it in the external editor. After you save it, the new saved version will automatically be used when you return to the Springboard file.
See also Using an external sound editor.