I've experienced this issue with my SBz using Optical out to a Denon AVR-4311ci setup for 5.1.
Depending on the driver being used, I switched the default Sound in the WINDOWS Control Panel to SPEAKER (under playback tab) rather than SPDIF-Out.
I have the *Sound Blaster Control Panel Utility* set to 5.1 under Speaker/Headphone Configuration. (I believe my Hero VI z87 SB Software is the same as the one used on my SBz)
This solved my problem. The problem occurred after a SB driver update.
Note, if you right click over Windows Sound Control Panel SPEAKERS or SPDIF-Out, you will have a menu option to configure the speakers. Select it to choose Stereo or 5.1 depending on your speaker setup.
If the above does not work, I would re-install the Sound Driver and/or SB Utility App to see if it resolves the issue.
Corsair 500R Case, H110 Hydro, 1200AX PSU, Asus Maximus Hero VI MB, Intel 4770K CPU, Gigabyte GPU GV-N98TWF3OC-6GD, G-Skill Trident 2400MHz 32GB, Crucial M500 960GB SSD, Seagate 6TB HDD x2, Creative SBZ to Denon AVR-4311ci - Infinity Primus 5.1 w/Klipsch Sub XW-300d, HP ZR30w 30" S-IPS LCD, W10 64bit