When opening a developers console in a browser, 404's appear to originate from within the plugin webapp:
GET https://xmpp.igniterealtime.org:7483/logo/conversejs-filled.svg 404 (Not Found) GET https://xmpp.igniterealtime.org:7483/sounds/msg_received.ogg 404 (Not Found)
These files probably need to be included from Converse's source code.