Use Flash 11's Sound::loadCompressedDataFromByteArray to load MP3 files

When available, use this method to load MP3s instead of our custom build-a-swf-in-memory method. For those with newer Flash versions this will fix https://github.com/LLK/scratch-flash/issues/206.
From the docs I see that loadCompressedDataFromByteArray can throw. Does it need to be wrapped in a try/catch or would it be safe to throw here?