Umm. To make things clear for the current situation.
Zyxel modem/router -> wireless eugenius router, without routing, so practically an AP -> 5p sitecom switch.
My assumption: The PC is connected to the switch. The SB to the AccessPoint.
The SB has internet (internetradio works) but does not find firefly from the PC. Right.
My guess: Multicast/Unicast problem. Try looking around in the eugenius admin page to enable something related to multicast/unicast.
Firefly relies on that to broadcast itself on the (wireless) network.