I assume you have you speakers hooked up via 3x2chan jacks right?
(front R/L, rear R/L and center/sub; green, black/grey and orange)
Right click sound icon, open 'playback devices'
select 'speakers' and click on 'configure'
select '5.1 surround' from list and click on next
check full-range Front/back if applicable (consult speaker specs to find out if your sattelites are full-range) and click on next
click on finish and close menus still open
This ensures windows has your speakers set up properly.
NOW, to the difficult part!
The fact that your music only plays over your front speakers is because that's the 2 channels they use. stereo music basically has 2 channels, and they will be outputted to your Front L/R jack UNLESS you tell your media player to do otherwise.
If you are using winamp/media player classic/zoomplayer for audio/video playback the solution requires installation of 'AC3 filter'.
Basically what this is is a codec filter that will 'take over' the audio handling from your player, decode it and send it to your speakers. You can configure it to convert your 2 channel stereo stream into a full-fledged 5.1 stream.
Here's a link to the site: AC3Filter
Install it, and restart if prompted to.
Now for instance you start winamp to play some music. What will happen is that a small orange-ish icon will appear on your iconbar in your lower right of screen.
Double click on it to open the AC3 Filter Properties
Here you will see under the 'MAIN' tab (loaded by default) on the left hand side a
box named "Output format"
in the listbox right under that select 3/2+SW 5.1 channels.
This will make AC3 filter turn ANY audiostream it receives into a 5.1 stream and send it to your speakers. You can see if its working by looking to the righthand side
at the "input levels" and "output levels".
If you are playing a stereo file the input levels should only show 2 green bars and the output level should show 5 or 6.
You can also select Dolby Surround/Dolby prologic or Dolby prologic II to make AC3 filter use the Dolby audio algorithm. It will have a very different effect than the previous mentioned setting. You should check both out and decide which you find best.
Sorry for this post being so long.. but I've had this problem myself aswell and I had to figure everything out on my own.
This WILL help you get 'proper' surround over your 5.1 speakers the way you want it to, I guarantee it!