I.e., just the audio. I have an ONKYO receiver that only supports audio streaming over DLNA, no video.
You can stream every kind of PC audio to a DLNA receiver with Stream What You Hear.
Stream What You Hear (SWYH) is a Windows application to broadcast the sound of your computer (ie: “what you hear”) on an UPnP/DLNA device such as TVs, amps, network receivers, game consoles, etc...
This functionality is now built-in to Microsofts Edge browser. Microsoft calls it "cast media to device" and it is "the new version of Play To".
In short, load the Youtube video or other media in an Edge tab, click the three-dots-button, then choose "cast media to device" and choose a connected device.
I've just come across this discussion, years later. For anyone else who comes via a search: as of March 2021 it is possible to cast to a DLNA-supporting device from Windows 10, with Microsoft Edge and with the Film & TV app. Also the Windows Explorer context menu has a Cast to Device option for supported file types. I am using an Arylic WiFi audio-only receiver. Somewhere in the menus of applications that can do this there is a "Cast to" option. Google Chrome can also cast, but probably only to a ChromeCast (it doesn't see my Arylic). I can't say whether it will also handle video - I don't see why not. I haven't yet been able to view video on a computer while casting the audio (I came here looking to solve that). Too late for the original poster, but I hope of use to someone.