you can very easily slow your fans down. just be sure that if you do it, that you make sure the cpu doesnt get to hot
fans are 12v, (the 3pin headers on the mobo are 12v) so you can easily unplug it from the mobo and hook it to one of the 4pin molex connectors from your powersupply. yellow is 12v (so attaching the wires to the black and yellow would make it the same speed as before) red is 5v (so attaching it to the black and red would slow it down a lot, probably to much)
or for some middle ground you can run it at 7v (which is what i often times use) you do that by grounding the fan to 5v (ie, attaching the black wire from the fan to the red one on the powersupply) and then the the other wire to the yellow 12v.