5.1 Game with 4.1 speaker setup

I'm looking to play Doom III and it says it supports 5.1 and that audio in that game is as important as graphics. I only have a 4.1 speaker setup for my PC. What happens now when I play Doom III? Will there be an option to output the sound in quadrophonic mode? Does the sound that's supposed to come out of the center channel get disrtributed to the front left, front right speakers? How big is the difference between 5.1 and 4.1?

Thanks!

The center channel sounds should be divided like you said, front left/right. The difference is very small.

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