think about this for a second.
music is stereo 2.0. youtube is at least mostly 2.0. most movies streaming online are 2.0. all streaming music is generally 2.0.
2.0 means stereo, or two speakers which explains why you are getting sound out of only two speakers. surround sound is generally at least 5.1 audio. my guess is that if you play movies with surround sound or games with surround sound you will get sound out of all speakers.
now, there is usually a way to get sound out of all speakers by duplicating the audio across them. if on windows 7 i know the below
If you're running Win7 and you have no other than the default sound driver, there is an option "Speaker Fill" that solves all problems!
First, right click on the sound icon in the task bar (right lower corner), choose "Playback Devices".
If you have only one sound output, it'll be the only one on the list. If you have more than one then, while playing some music, you'll notice one of the device's volume bar moving, that's the one you should select.
Select your audio input and click on "Properties". Then go to "Enhacements" tab. There should be a checkbox with the "Speaker Fill" in the middle. Select it and press apply.
keep in mind if you play 5.1 content after this it may not work right and you may have to disable/enable this option as you see fit.