fix(levels): set tranquil-wave default volume to 0.5

Author: remarkablemark

src/levels/tranquil-wave.json

@@ -5,7 +5,7 @@
   "difficulty": "hard",
   "background": "sprites/background/GuitarCity.png",
   "music": ["sounds/tranquil-wave.webm", "sounds/tranquil-wave.mp3"],
-  "volume": 0.0,
+  "volume": 0.5,
   "directions": [
     { "direction": "right", "time": 7.5, "duration": 3 },
     { "direction": "left", "time": 7.5, "duration": 3 },