192.168.*.* is a private network IP so your router is doing NAT. UDP does not work through NAT without other tricks like port forwarding, STUN, or ICE. The software you're using on your laptop or PC might be using STUN or ICE or setting rules on your router via UPnP/Bonjour. I don't think that mplayer supports these options.

Your best bet might be to set up port forwarding on the router for the UDP packets to your n900.