Load audio from folder outside build directory

[jakory]

Like #3, currently, you can only load or playback audio in an Opal game if the audio files are already in the Unity build directory (e.g., with the game assets under Resources).

However, it would very useful to be able to load or playback audio from an arbitrary folder on the device, since then, you could make a folder of audio files outside the app that you can easily add to or modify without recompiling the whole app.