So I tried now stream over http with vlc and using the mediaplayer on n900 (internetradio).
I used wifi network but should work with usb network too and it has 3 seconds delay.

However to get all windows sound streamed you need for example icecast which you can easy configure with altacast and stream over http the stereomixer sound in ogg format. For the N900 you need to install ogg-support and open mediaplayer....
This method has a 13 seconds delay.