I had the exact same issue and pressed iTunes for a solution and after really engaging with them, they came back and after testing what they suggested I now have answer that works. As with all things all it needed was to know which box to tick and the important thing to note is the solution lies not in iTunes but in a simple adjustment to your Windows 10 sound settings.
SizzlingKeys 5.1.2 – iTunes controller. Keygen Music SizzlingKeys. SizzlingKeys Lets you control iTunes without leaving your current application. Control all aspects of iTunes with hot keys. That are fully-customizable via an intuitive user interface.
So the solution is this:
First close iTunes (if open) then go to Windows Control Panel - Sounds - Select your speakers (probably you defaulted device) then click on Properties button. Next click on Enhancements Tab and simply tick the Speaker Fill checkbox - click on Apply and can test to see if working by using Preview button. Surround sound on all 5.1 speakers should play.
If so next open iTunes and play your favourite song and it should play in all 5.1 surround sound.

Hope this works for you - it did for me and is a confirmed solution by iTunes Support.
If it does work for you spread the word on this solution.
PS I believe this also may resolve / solve 5.1 surround sound issues for iMovies and other similar sound issues.
Enjoy
Mike H
Feb 16, 2016 8:24 AM
If you use iTunes to manage your music collection, you will no doubt have been frustrated by the fact that it is necessary to switch to from the program you are working with back to iTunes to switch to another track or pause playback. SizzlingKeys is a simple little app that makes it possible to do just this no matter which program you are using.
SizzlingKeys installs as a preferences panel and it allow for the creation of customisable keyboard shortcuts to access common iTunes controls. In terms of playback, you have the option of pausing and unpausing playback, skipping back and forth between tracks and adjusting volume.
You can also mute and quieten playback with a simple key press and opt to show or hide iTunes. Access to playlists and searches is also just a key pres away. The currently playing track can rated with iTunes' star system as well.
SizzlingKeys can be configured to launch at system startup so iTunes can be controlled straight away and while the tool is being used, the actions you perform are accompanied by a translucent floating status panel. The program is simple and effective, and makes working with iTunes far more enjoyable.
Changelog
SizzlingKeys sounds like a very simple tool, which it is, but it will quickly become an essential component of your Mac's setup.