You can project from one Win10 device to another since the Anniversary update without any particular hassle. (Nathan mentioned this some months ago and I just got time today to check it out)
To my delight, the sound works insta-magically and comes out of the device you are projecting to.
Why is this so great? if you have an ultra portable or win tablet and want more screen real estate you can use project and choose to project as a duplicate or extended. So I can use my SP4 in conjunction with any other laptop, desktop or screen hooked up to another PC wirelessly and use it's monitor even if the monitor device itself (like my dumb tv) doesn't support miracast.
This would be even more awesome if it worked with my Toshiba Tecra r850 which is hooked up to my (too dumb for miracast) TV, but it does work with my Asus laptop which is hooked up to an external monitor. The Toshiba looks like it should work but when I try to download widi software for it, I get a not compatible error. Sigh. I may just switch my Asus to the TV because it sure would be nice to project from my SP4 to the TV to use it as a great big extra monitor without cables!
Here is a link on how to do it:
http://www.laptopmag.com/articles/turn-windows-10-pc-into-wirelss-display
I tried it out by running a Youtube video on my SP4 projected to my Asus laptop (about three years old) which was itself hooked up to an external monitor via a cable.

