Check the ownership of the files located in the mp3 directory.
If they aren't owned by the account user then they will not count towards the used space. The WHM/Cpanel count only includes files owned by the user.
Most likely these files were uploaded via a script, and if they were they will take the same owner details from under which the script was run. If you are not running suexec or phpsuexec then the user will be 'nobody' (the user under which Apache runs).
There's a couple of things you can do to get around this.
Either enable suexec and phpsuexec so that all scripts run under the user (not an option to consider lightly - be sure to read up on the pros and cons) or set a cron job to periodically (as you see fit) change the ownership of files in this directory to that of the account owner.