Why my Routeur Netgear DG834G doesn't allow to share my ITunes library ?

Firewall rules usually only apply to the WAN port so unless you have configured the router otherwise, I would close that port.

The one common item to all of your problems is mDNS or Bonjour. That is what Macs use to advertise all of the services you listed. If all devices are on the same subnet, it should work with out any problems unless you have configured the router to filter multicasts.

Are your wired and wireless clients on the same subnet? If they are not on the same subnet, your router would have to route multicasts. Though most "home" routers only route with regard to the WAN port, so I would look for some kind of filter that is stopping multicasts on the local network.

I don't know enough about that router to say what it normally does with multicasts.

Before we get into that, make sure your router has the latest firmware from Netgear. The DG834Gv3 model had a known problem with passing multicasts between the wired and wireless networks. The update for this was released last month. Check the Netgear website and confirm which version of that model you have and if you need the firmware update.

http://kbserver.netgear.com/products/DG834G.asp