Thanks for your reply. Will probably be getting the sound card anyway (cheap so meh) - Tried it on my other computer and exactly the same problem, bearing in mind that it is really old/slow. Not sure if this means it might be an actual problem with the driver in the ear or not...
Okay, late reply here, finally got my soundcard (late delivery and such).
Didn't solve this problem at all. (Got a Xonar DGX) But it has really reduced the interference I was getting when I had 5.1 selected. (Been using stereo for a while, which is annoying with only one side working, lol.)
Not sure if there is a fix for this, might just be a problem with the headset, feel like getting a proper set of headphones to replace these anyway, not sure.