No you're right they should be playing. In Windows 7 the black jack was actually used as rear's. Having a look at your motherboard for 6 speaker configuration (they describe sub as 6th speaker) in Win8.1 and 10 the blue jack is used for side pairs instead of black / rears.
Im still on Win7 but i had this happen when i trialed Win10 and couldn't get the rears to work using the black jack. My 2600k mobo has grey jack for 7.1 surround but Win10 or Realtek changed it so i had to use grey for 5.1 rear pairs. In your case is gone from black to blue. Were you using an older os (Win7) on your older board?
Just some descriptions regarding options in Sonic studio
https/www.asus.com/au/support/FAQ/1015213
Windows and Realtek Audio manager used in conjunction with Sonic studio, the speaker fill option enables all surround speakers to play stereo sounds.
Also some things on VLC player, https
/www.addictivetips.com/windows-tips/how-to-set-default-audio-output-to-5-1-surround-sound-in-vlc-player/
VLC you've probably had a look how to set 5.1 for stereo content, similar to virtual prologic. Might need to toggle Float32 output.
So to recap, reinstall all audio software for your motherboard. Use green (front) orange (voice / sub) and blue for side pairs. In the speaker setup make sure side pairs is checked and not rears. Sides are now rears in Win8.1/Win10 for 5.1 setups.
Don't forget to try speaker fill