I managed to do this by using vlc.
In the PC console type:

vlc movie.avi --sout '#duplicate{dst=display{delay=1400},dst=udp{mux=ts ,caching=0,dst=192.168.0.5:2010},select=audio}'

where movie.avi is the video to be played, and 192.168.0.5 is the address of N900. This will show the video on the PC and stream the audio through the LAN. Then in N900 terminal type:

mplayer udp://@:2010

to play the audio on the N900. You can use +/- keys to sync audio delay if necessary. I also use bluemaemo to control vlc from my N900