Please wait a full 24 hours before bumping your topics.
Also your question is actually pretty much out of the scope of Construct 2. At least the answer is.
HTML5 won't allow you to upload files to the server it is being hosted on. So this is not handled from within your game.
Now nothing prevents you from using a server script to put files in the "Media" folder, and find a way to let your application know that it needs to play a file "by name" with the new name of the file you uploaded.
Perhaps you need to keep a list of the names in a file and have AJAX calls from your game to read them, and your server script to modify that list.
For everything server-side, this is out of the scope of Construct 2 and these forums so you'll better be asking on online communitites dedicated to the script language you've chosen.
For AJAX, check the How do I FAQ for example of use.