There are a number of DLAN devices, but I am going to assume that you are using a Powerline-like device to connect your Mac back to your router. As you know Powerline devices are used to create a pseudo Ethernet connection using your home's electrical wiring. If this is what you are referring to, then you can share this connection via the Mac's wireless adapter. However, you cannot extend this wireless network with an AirPort base station.
For iTunes streaming, you need either a wired or wireless network for the iTunes host to stream to an AirPlay speaker. That speaker could be an AirPort Express base station, Apple TV, or a device that supports AirPlay.
Thanks for your response, most grateful. Yes, I have the Devolo Powerline-like network which works great for hard connection to Mac, Apple TV and Xbox.
I've not been so lucky with my AEs though. So if the Mac is psuedo Ethernet-connected to the router, it then communicates with the AEs via the Mac's wireless connection?
If the AEs are out of range of my Mac, can I connect one of them to a Devolo device and then extend the network (or direct connect) to my powered speakers, or is there another way I can use my AEs on powered speakers distributed through the house?