This has been discussed many times. If you use a servername.subsonic.org address, you cannot hide the ip address as the service is a simple redirect service that does not use DNS. When you type in the subsonic name it just translates the request into your IP address and port number to send the request to your server.
You can avoid this if it's really important by using your own domain name that uses dns. I have an account through my Windows Home server the allows me to use servername.homeserver.com for accessing the remote web page of the server. If I type myservername.homeserver.com:4040 it will make the connection to that subsonic instance using dns and port forwarding and then the address line will retain the dns name instead of the ip address.
It really doesn't matter because you can reverse lookup and domain name and get the ip address so other than aesthetics, there's no real benefit to one over the other.
There are also dns services available that can allow you to buy your own domain and use whatever you like, but if you use the subsonic redirection service the address bar will always show the ip address.
http://www.selfseo.com/find_ip_address_of_a_website.phpType in any url and it will tell you the ip address. If anyone has enough moxie to try and hack your site they would certainly know how easy it is to obtain your ip address regardless of the domain name you use. There is no added security in hiding your ip address.