Fixing audio resources and fixing node deprecation warnings

In debugging the issue that the local install doesn't proceed past the "Connecting to server..." screen, I noticed that the included audio files were present, and were being reference from the client/audio/music directory. I changed the code to use the correct directory and downloaded and added the files from the Mozilla CDN.

I also fixed the deprecation warnings that node throws when using the path.exists and path.existsSync methods.