this is because it uses

dynamic port with port addressing ranges from a few thousands to tens thousands (I can't recall the exact range). So, port-forwarding will not solve your problem. If your router support UPNP, then you might be able to get audio connect. Most yahoo DSL users use the 2Wire Homeportal router which doesn't support UPNP. Thus MSN messenger report non-UPNP IP restricted NAT. On an non-UPNP router, you can try assigning your machine to the DMZ mode which simulates direct connection to the internet with all ports open for communication. I hope that you can resolve your problem.