Assuming Ron is on the right track with the File Not Found (error in the log you posted) – and of course there is no reason to think he isn’t – I don’t know if changing your Soundbridge firmware version is going to resolve the problem. I am running the latest Soundbridge firmware with the latest Firefly nightly on a NSLU2 and everything is working perfectly for me for example.
Do you have a PC or Mac with iTunes on it? If so, can you try playing the same track(s) from Firefly (should show up as a Shared Library in iTunes) via iTunes and see if they succeed that way. If iTunes also fails it’ll likely be some funny setup peculiarity with your Firefly.