I find it very strange that it’s omitted, but to deroute the microphone cable, you will have to unscrew the cable right next to (above) it. At least, I had to. It’s not hard, but you can’t deroute the microphone cable without derouting the other cable.
Make sure to remember that this screw is removed when reassembling!
I have the regular (not Santa Rosa/Penryn) model, and instead of a 6mm Phillips screw on the left, it was a 5mm Phillips screw. It didn’t make a difference, however.
In my macbook, which is the regular (not Santa Rosa/Penryn) model, the longest 10mm screw is the same, but the middle screw and the right screw are both approximately 5mm. However, the middle screw is fatter/wider and the right screw is thinner.
Also, when reassembling, make sure all three screws are easily screwed in before tightening each one. The hinge mount that you’re fastening down will wiggle a bit before being fully tightened down, possibly causing the screw holes to not line up!