After some private messaging with the author, we found the problems and have them fixed. Other users may be interested in case they run into problems too.
The first thing users need to be aware of is that unlike the FirePlay client, part of this runs locally wherever you’re listening. Thus if you plan on listening at a machine other than where FireFly is running, one of the files needs to be modified to the specific IP of the server, rather than being left as localhost. There’s a line in SilverPlay.html that needs to be changed.
Remove “localhost” and insert either the server/host IP address or its domain. In my case I use myid.dyndns.org. Also, if you run FireFly on a non-standard port, change the port number.
Linux users need to make another change due to linux treating filenames as case sensitive. ClientAccessPolicy.xml in the web root folder should be changed to all lower case. The author indicated he will fix that in upcoming distributions so this step won’t be necessary.
After making those changes, I have access from both inside my local network and outside on the web.
As for first impressions: I have however experienced some hiccups in playback even after a song is fully buffered. Not sure if that’s in Silverlight, or a glitch in this implementation of it. In comparison to FirePlay, SilverPlay is prettier, but has an inherent disadvantage in that it requires the Silverlight program be installed on the machine where you’re listening. You won’t be able to sit down at a friend’s computer and start playing music right away like you can with FirePlay.