You are using MC on a Windows PC and want to stream audio to an Airport Express, and use the audio out on it?
MC for Windows outputs to an attached audio device, or a networked DLNA device. Can Windows stream to the Airport Express? Does Windows see the Airport Express as an audio device? There is no mention of DLNA on the Wikipedia page:
https://en.wikipedia.org/wiki/AirPort_ExpressYou can use utilities as mentioned on that page. That gets over the issue of Apple proprietary technology and DRM, I think.
But I don't use Apple products anymore, and don't have an Airport Express. I only answered because this is the third time you have asked the same question. I guess nobody else knows either.
If no one else answers, I think Google is your friend. Any answers explaining how to stream audio fro Windows to an Airport Express would probably be applicable to JRiver MC.