The built-in Media Player is a DLNA/UPnP renderer - if you have a UPnP server on your network, it should show up in the media player and you can play your media from there.

(Unfortunately, unlike the preceding N8x0s, this is only a renderer, it can't be used as a control point to control what's playing on your other UPnP devices.)

If you want to set your N900 up permanently attached to your stereo, you should be able to control it remotely using VNC (running the X11VNC server on the N900 and a VNC client on your PC).