It just came to my mind, why it was silent:
The audio files simply weren’t loaded immediately. That’s all! 🙂
If you change downloading into a real streaming, as you wrote before, you’ll problably overcome this behavior.
However, there unfortunately is the other sound/music bug left, you also could see (or better hear) in that video I already shared with you. This bug makes the engine not to not change sound/music when the hero enters a new level.