macOS Sierra 2.1 to 5.1 upmixer? Is there any way to play music through all of the speakers in a 5.1 (or 7.1) surround system in Sierra? I've looked at options like Soundflower, but everything I've looked into is 3+ years old.

To clarify:

I'm looking for a way to play 2.1 sound out of all speakers on a 5.1 surround sound system. I have already configured my 5.1 speakers properly, I do not need help with that. For Windows, there is a handy program called Speaker Fill, but it is not available for Mac.

Below are screenshots of my audio configuration. The 5.1 surround it connected via a USB Sound Card. I have verified that all 5 speakers are connected to the correct channels via the test in MIDI Setup.
Yes, you can do what you want with Vox Music Player and maybe also Jack OS X.

In terms of which option you'd be better using, it really gets down to what you want to do? If you just want to be able to play music, Vox Music Player is the easier option (it's confirmed as macOS Sierra compatible).

However, if you want to use all your speakers for other uses and/or want the ability to control which applications do and don't use your speakers, then Jack OS X provides more flexibility in that regard (however I have not used it with macOS Sierra - it does, however, do what you want on earlier systems).

Unless something has changed, both options are free to download.

If you do want to try using the Jack OS X option, use the instructions below. 

INSTRUCTIONS FOR JACK OS X

I have not tested these steps in macOS Sierra, so please be sure to report back with how you go as it may help others.


Download Jack OS X (be sure to download the appropriate version)
Install Jack (Note: To be able to actually install the software, you may need to manually go to the download and press the Control key while clicking on the app icon and then choose Open from the shortcut menu. Then, when the message appears, click on Open.)
After installation, reboot your Mac
Open JackPilot from within the Jack folder (This should be located in your Applications folder)
In Preferences set the Interface to match how it is your speakers are connected (e.g. USB sound card)
Ensure that both the virtual inputs and outputs match the Interface you selected at Step 4
Now go back to Finder and launch the Audio MIDI Setup app (This should be located in your Applications/Utilities folder)
Select the properties for JackRouter (Note: If you don’t see JackRouter, make sure you have Started it in Jack)
Click on Configure Speakers…
Set your speaker outputs by using the drop-downs to set the output for each speaker. (Note: You can click the speaker name to do a sound test for that speaker). 
Click on Done
Open iTunes and play some music
Go to JackPilot and click on Start
Click on Routing (may take a little while to appear)
Choose iTunes by pressing the drop-down arrow
You should see some outputs labelled as out1, out2 etc. Choose out1.
In the second column, use the drop-down arrow for System and double-click on the option that applies to you (e.g. double-click on playback_2,3,6 (where 2 and 6 represent side speakers, 3 represents a subwoofer)
For out2 choose the remaining playbacks. (Note: Do not choose any you didn’t select in Audio Midi Setup earlier at step 6 - 10. 


NOTE: Where I refer to iTunes at steps 12 and 15, you can select other apps instead (including games etc).