@Override
                    public void onDataIsReady() {
                        WritableMap returnMap = Arguments.createMap();
                        returnMap.putString("key", key);
                        returnMap.putBoolean("result",true);
                        returnMap.putString("msg","");
                        onSoundPreloadedCallback.resolve(returnMap);
                    }
This function takes a map of keys to sounds and creates SoundData objects
to prefetch the data before storing them in a local map. We currently only
support external urls (web-based).

@param soundMap a map of String keys to String url's.
android/viro_bridge/src/main/java/com/viromedia/bridge/module/SoundModule.java