Apple has been providing a support for installing Microsoft Windows operating systems on all Apple hardware equipped with an Intel processor through Boot Camp. But hardware compatibility issues sometimes are very annoying.
The MacBook and MacBook Pro CirrusLogic sound chip are notorious for unsupportive sound drivers in Boot Camp 3 and 3.1. The internal microphone is not working in these machines and the volume of the loudspeakers is painfully low.
There is no official solution to the problem but Luca has proposed a work-around (that worked for me). His first solution is not working but the second one works like charm. It involves downloading the previous version of Boot Camp and extract the audio driver to update the current drivers.