Why do I only get sound from the left and right front speakers?

go to your sound settings in windows and ensure that all speakers are enabled and do the sound test.  Afterwards you need 5.1 sound source to get 5.1 sound.

You need to connect your speakers to Line out 1 through line out 3 and make sure Surround mode is selected from sound blaster settings.
You also may need to disable onboard sound.

When I do the sound test in Windows, it appears to test all speakers, but sound only comes from the front speakers.   Not sure how to disable onboard sound.   Is it disables in Setup?   I have set up the Sound Blaster software per Ramin's illustration.   When I test the speakers it appears to test all, as  in Windows, but sound only comes from the front speakers.

Does it matter whether I use the optical connection versus the 6 connection?   I have tried both, with the same result.

If your connections are the same with below picture then you have to Enable DTS / Dolby surround from your Settings. it needs a software setting.

When I try to enable DST or Dolby I get this message:

Sound Blaster Z-Series Control Panel
The SPDIF-Out (Sound Blaster Z) device is being used by another application.   Please close any applications that may be playing audio to this device and try again.

I do not know of any other application that may be using this.   What should I look for?

Check your BIOS for onboard Sound and Disable it.

Or uninstall sound blaster Software and driver, Restart the PC and reinstall the software and driver.

I disabled Sound in the BIOS, it did not help.   I will uninstall the Sound Blaster software.   Do I need to physically remove the sound card as well?

Just to be sure you can re-seat it. but it's not necessary.

I re-seated the card.   Still only sound from front speakers.

In Windows Sound, there are two items, Speakers Sound Blaster Z Ready and SPDIF-Out Sound Blaster Z Default Device.

Thanks,  and I'm glad we could help.