You can look here for themes or try doing a search in google for phpnuke themes or free phpnuke themes.

As for creating a music website, if you are thinking of hosting and/or uploading music to your webserver and making it available on your site, you may want to clear it with your webhost. Many webhosts don't allow you to host music on their servers due to copyright issues and it could end in your account being suspended. If in doubt, look at your webhosts Terms and Conditions or contact Customer Support.