iMac (on Ethernet) Can No Longer Stream to Airport Express (on WiFi)

After years of reliable service, all of a sudden I cannot stream music from iTunes on my iMac, via my Airport Express, to my stereo system.

First, a bit about my configuration:

* iMac connected via Ethernet to a wireless router (D-Link DIR-655).

* iMac Airport (WiFi) is turned off.

* D-Link sets up a wireless network, and Airport Express joins the network.

* Airport Express does not set up its own network, it's simply a client of the existing D-Link wireless network.

* iTunes "sees" the Airport Express, and can stream audio via AirPlay.

* iPad also joins the wireless network, and also can stream to the Airport Express via AirPlay.

* iMac running 10.6.8, with all software updates applied.

* Airport Express running latest firmware (7.6.1).

Well, that was a few days ago, when "it just worked". But for some unknown reason iTunes could no longer "see" the Airport Express.

So I took my Airport Express to the Apple Store and they did a Factory Reset, but it did not solve the problem. I figured my rather old Airport Express was failing, so today I bought a new one. The symptoms have changed a bit, but even after spending an hour on the phone with AppleCare, it doesn't work properly.

Now, the only way the iMac can stream to the Airport Express is if the iMac also joins the wireless network. In fact, now with Airport turned OFF on the iMac, the Airport Utility cannot see the Airport Express. But here's another weird thing: when I use the IP address of the Airport Express, the Airport Utility CAN connect with and manage the Airport Express, even with Airport turned OFF on the iMac!

I should also mention, the iPad has no issues whatsoever connecting to the D-Link's wireless network and streaming music to the Airport Express.

This all seems like something might have changed on the D-Link wireless router. But for the life of me, I cannot figure out what setting(s) on the router might allow the iMac - over Ethernet - to stream audio to the Airport Express.

Another thing. When I have Airport turned on on the iMac, it can "see" and stream to the Airport Express, but the music has very frequent drop-outs. Could this be related to the fact that the iMac has two IP addresses (one on Ethernet, one on WiFi)?

My goal is to get things "back to normal", where I can turn off Airport on the iMac, and all LAN and WAN traffic from the iMac goes over Ethernet. I have a few questions (for now :-)

* Since my iMac has two connections to my LAN using two IP addresses (Ethernet & WiFi), is that likely to cause any issues?

* When two computers join the same wireless LAN, can they effectively talk directly to one another, or does the wireless router need to be involved every time one computer sends a TCP or UDP packet to the other?

* Do routers (or the DIR-655 in particular) tend to have any firewalls or policies that control connections on the LAN between Ethernet and WiFi clients?

* Since AirPlay uses TCP port 3689 and UDP port 5353, would the DIR-655 need any special configuration to "open these ports" so an Ethernet client can connect with a WiFi client?

So, any help would be most appreciated!

Thanks!

Jim