Hm, sounds familiar. I guess the first thing you could do is see if the same thing happens with a different speaker set.
If it happens again with a different set then it MIGHT be the sound card, but doubtful.
If my instinct is worth anything, then I suspect it may have something to do with either the power source and the ground, or unshielded cables. (Here's hoping the Dutch and English terms are the same. )
Aye, tried it with a different speaker set. As I mentioned in my post, this occurs with my 5.1 speakers, but I can switch to a 2.1 set which work just fine and the problem never occurs with them.
Not sure about unshielded cables or the power source - speakers aren't my area of expertise. Given that I can use another set of speakers without a problem I don't think it's the sound card, but I'm not ruling anything out at the moment. I haven't tried it with another 5.1 set (given that I don't own another).
As I also mentioned, when I plug the 5.1 speakers back in after (usually) a week or two, they work fine for a week, so no clue what's going on there.