Maybe ... let's go over the you-must-be-kidding stuff first.

1) Is the computer powered on?
2) Does the computer have a sound card?
3) Are the speakers plugged in to the sound card, powered (if required), and
turned on?
4) Is the microphone plugged into the correct hole on the sound card?
5) Is the volume turned up?
6) Do other sounds play from the computer over the speakers?
7) Can you record anything into a sound file from the computer microphone?

... if these are all Yes, then we can move on ....

8) What the microphone muted or turned down during the recording?
9) Do any sounds inserted into PowerPoint play?
10) What is the full pathname and filename  length to your presentation?
       C:\MyPresentation.ppt = 21
11) Did you adjust the sound quality in PowerPoint  prior to recording?
12) What steps (as exactly as you can) did you follow to insert the sounds?