Is there a way to use my line-in as a line-out for my speakers?

I doubt it...Laptops weren't designed to do 5.1 (unless it specifically states that it does...in that case you would have 3 specific out jacks) the out jack is for headphones the in jack is for a mic. the best you can do is two speakers...but that to may be a problem...doubling up on speakers also doubles up the ohm age so your sound my not take it or as best be very low. There are external speakers for laptops and you may find an amp (Amplifier) that can take your low level output and be able to use more speakers...even convert (sort of) to 5.1