YouTube (and Vimeo) have changed what and how they allow access to content. You have to go through their API, which works differently from methods described in this thread. You can search “YouTube” on this forum to get more info.
I’ve found the Internet Archive is a good place to freely host audio and video files, as well as get tons of content for just about anything. You can get a URL which will quickly load your sound file, and choose to make it publicly available or not.
Another approach I’ve used for local-only files called from Axure prototypes is to put them in a folder at same level as or one folder above my HTML folder, e.g., named “sounds”. Then, just zip up that folder with your HTML folder and port it anywhere. In this way, you can keep updating and publishing your Axure prototype without having to keep moving files around, as well as reuse those sound files in other prototypes.
Pro Tip: It’s best to use relative URLs in your Axure calls, e.g., “…/sounds/myfile.mp3” instead of “C:\otherfolder\sounds\myfile.mp3” to make it more portable.