Depending on your version of PowerPoint, you may have trouble using the native recording function and getting a file that is small enough to send on the web. If you are seeing large files sizes (larger 35MB), you should probably record your audio separately and import to PowerPoint. The Mac version of PowerPoint in particular has issues with large filesizes with natively recorded audio.
The video below shows you how to record using Vocaroo, a website that uses Flash to record your audio. Note that if you’re importing audio into PowerPoint, you’ll need a separate audio file for each slide.
Note that if you want more advanced editing features, Audacity is a fantastic program but it requires a bit more work to install.
Once you have your audio recorded, you would then need to insert the file into your PowerPoint presentation.