I'm using the playTrack() command with a locally hosted file and it works fine for me:
I didn't even use port 8080 like I probably should have. 
I'm not sure why yours isn't working, but I'd probably start by leaving Rule Machine out of the picture and seeing what works if you try running the command with this parameter (or another appropriately formatted command) from the device page directly.
When you find something that works, bringing into RM should be easy, but this would at least rule out RM as a source of the problem. It works for me both ways, and the only difference I can find from your description is that I'm using a WAV and not an MP3. I don't really know what formats are supported, but I suppose that's something else you could experiment with.
