You can actually use a WAV sound file as your buddy icon, but instead of displaying an image, it will play the sound.  To do this, change your buddy icon as normal, but when the window comes up asking you to select a buddy icon file, type "*wav" and hit enter.

This will display all the WAV files in the directory you are in.  Browse your computer and find the WAV file you want to use as your buddy icon, select it, click "OK" and then close your preferences window.  The next time you IM someone they will hear the WAV file you selected play.

Notes: The WAV file MUST be less then 7168 bytes otherwise AIM won't let you load it as your buddy icon.