5.1 works in sound test, not in playback

[Muppet Rat]

***SOLVED*** See my last post.

I've set up my new 5.1 system and it's driving me crazy. If I test it using the playback devices>speakers>configure option, it works fine. If I test it using any other means (I've been trying lots of different youtube test vids) it just doesn't work. The results vary, but mostly I don't get rear sound (it diverts to the front) or it comes from FL fine, then both C&FR at the same time.
The way I have it set up is a little confusing too.
Sub to TV is via optical.
Sub to PC is using RCA-3.5mm. I'm using on board sound and connecting to the green, black & yellow 3.5mm.
I'm at my wits end here, please help 🙁

 

[PsyKhiqZero]

Have you tried testing you DVD playback? I don't know of many Yoututbe vids that are done in 5.1 surround. Also if you have a game that has has 5.1 surround sound as an option then test that too.

 

[Muppet Rat]

I've also been using some online tests (not youtube) and I'm getting the same results. The optical connection to the TV/BR is fine, it's just my PC connection that isn't working.

 

[Muppet Rat]

I think I've fixed it. I was able to d/l an audio driver from the MB site and it has an audio manager included. After fixing the settings in this, the test worked fine (but I wasn't convinced as the other test was fine before). I then d/l'd a test file from here and that worked fine too! So I thing I've cracked it!
Thanks for the help Psykhiq, I do think I was getting erroneous results from the other tests (especially after reading about simulated SS vs true SS).
I will mark this as solved to aid anyone else with a similar problem.