The easiest way is going to be burning the WMA file onto a CD as an audio CD not a data CD and then re-importing the newly created audio CD into iTunes. You will need Windows Media Player to create the audio CD.
Have you tried any other audio devices such as USB I/O? Sometimes changing the audio system properties to another I/O will jog the machine into recognizing the built-ins again.
I personally look at quality. I would rather pay more for high quality accessories than save some money and have to replace them in six months. Original Apple accessories are almost always high quality. There are also other great brands out there, but quality is my number one consideration.
Two options, buy MS Office for Mac. I did this for a long time and still do when necessary. Option 2 is to try using Google docs to create your assignments and then download them as MS Office files.