Finally got it, I think. The path to the end.ogg file was constructed differently from other sounds loaded by libstormgames. Because the libstormgames version works, just use it here too.

wicked_quest.py

@@ -445,10 +445,9 @@ class WickedQuest:
                     # Game complete - use gameStartTime for total
                     totalTime = pygame.time.get_ticks() - self.gameStartTime
                     if self.player.xPos >= self.currentLevel.rightBoundary:
-                        # Check for end of game scene
-                        gamePath = get_game_dir_path(self.currentGame)
+                        # Check for end of game scene using relative path like other sounds
                         for ext in ['.wav', '.ogg', '.mp3']:
-                            endFile = os.path.join(gamePath, f'end{ext}')
+                            endFile = os.path.join("levels", self.currentGame, f'end{ext}')
                             if os.path.exists(endFile):
                                 self.get_sounds()['end_scene'] = pygame.mixer.Sound(endFile)
                                 cut_scene(self.get_sounds(), 'end_scene')