I'm using madsonic (Build 5200) behind a mod_proxy configuration and had some troubles because there a wrong references where you're missing the prefix (part after the fqdn). Should break if you're not running madsonic in your web root (/). I detected it because the rest of my webtree is protected by http basic auth.
Happens after login, on genre tab, on settings tab, ...
Some wrong references:
/icons/default/arrow-all.png from /style/jquery.dropdownReplacement.css
/script/fontawesome/css/font-awesome.min.css from adminSettings.view
/script/jquery-migrate-1.2.1.js from loadPlaylist.view