It’s almost certainly a multicast issue, or related to avahi/mDNS. (Thats the software part sendin the multicast packets)
Read wiki to see what a multicast is: http://en.wikipedia.org/wiki/IP_Multicast
If you’re so convinced that firefly’s not the culprit try some other daap-compliant software such as banshee, rhythmbox, amarok.
Given you’ve got a RoKu unit, and not a pinnacle one that is.
Other than that you can also use uPNP servers like mediatomb or fuppes to serve music to the soundbridge.
So you see there are quite a few options, really.