Well, if your soundbridge is indeed a RoKu then it should see the iTunes, rhythmbox, banshee and firefly shares anywho.
So let’s take this one step further.
How is your RoKu connected to your music servers? Wirelessly? Via a Router?
Chances are, the router blocks multicasts, so see if you can tap into your router config and activate any options that enable multicasts.
As a workaround, wire up your soundbridge to the network and see if its working then, then you’ve found the culprit 😉