PowerPoint won't do it for U.
You will have to use an external sound editor,
but if your question was regarding how to extract the wav file from PPT then
you can save the ppt as html file and the you will be able to locate the wav
file there and then use any sound editor to convert it into MP3
- Abhishek
authorGEN - PowerPoint to Flash in just a single Click
http://www.authorgen.com