Another issue looks like you should fix, is prevent directory listing. Login into cPanel go down to the bottom under Advanced section, and click on Index Manager. Click okay to select the default folder. From the folder tree click on your audio folder. On the following screen, choose "No Indexing" radio box and hit okay. Right now if you type in your address pointing to the audio folder, all of the folders and files are shown. This allows anyone to be able to download or access all of the content under this folder.
Or you can upload a blank index.html file into all of your subdirectories, so the files are not shown.
In regards to using .htaccess, it seems you are on the right track. Check if your audio folder also has a .htaccess file as subdirectories override the one in your webroot.
If you can't get it working using .htaccess, you may consider using php and mysql database to handle the process. For I site I am working on, a user can upload a file (ie. image or mp3). The files are uploaded to random folders and added to a database. I use a php script that calls a file id (ie. http://www.natilie-brown.com/mp3/1), checks the user/referrer, gets the file location from the DB, and then streams the file to the browser. It would be very difficult for a user to ever guess the real filename and real directory to hotlink the file.
This approach is transparent to the user, and offers a level of authentication.