I found this in a few different places around the 'net - hopefully it's helpful:

Add a Voice Message to an E-Mail Message

Personalize e-mail with a voice message? You can perform this task using the Windows XP Sound Recorder and a microphone. First, open the sound recorder:

  • Click Start, click Programs, click Accessories, click Entertainment, and then click Sound Recorder.
  • On the File menu, click New.
  • To begin recording, click Record, and to stop recording, click Stop.
  • Save the .wav file to a folder of your choice.
  • Attach the file to an e-mail message.

To record sounds, your computer must be equipped with a microphone. Recorded sounds are saved as waveform (.wav) files.