[Hack&Lube]

This is a software problem, not hardware. Stop messing with all these soundcards and figure out how to get your audio setup properly done through the audio configuration for the onboard sound or whatever you have. This should work regardless of your hardware.

5.1 won't work properly if the original audio source is not encoded beyond standard stereo. It will not know how to split it up because the information is not in the audio stream.

To deal with this, you can activate the Dolby or Virtual Surround options in the audio configuration software which simulate 5.1 from a stereo source. Your songs from iTunes are only in stereo so your soundcard is defaulting to the correct setting by outputting through your front left and right only as these sources are not encoded in 5.1 (where center is usually for speech and rear left and right are for rear surround). To change this, you need to override the settings in the speaker and soundcard configuration or using a virtual surround option in software.