A very free method, although it would require a certain amount of interactivity from the author as would dedicated software you ask about, would be to send a sound clip as your message in the form of an attachment.

Without using 3rd party recording applications you could use Microsoft's own 'Sound Recorder' located in Start > All Programs > Accessories > Entertainment > Sound Recorder.

Simply press the Red button to record your message. A microphone is required to do this.
Stop is by the Black button.

Save As via the File menu in a location of your choice.
All 'Sound Clips' are saved in a WAV format.

To send this as an attachment, open your email client and input the relevant sending fields and insert the sound clip as an attachment.

The recipient will receive the clip and be able to open it in their chosen media player.

The recorder by default only allows 60 seconds of recording but have a look at this link for a tip on how to extend the time . . . >> To add more time to the recording session

Cheap but cheerful and effective.

Happy New Year.