This idea comes to mind:

Preparations:

1) Add the sound you want to play back to your iTunes library 
2) Create a new iTunes playlist with just that sound in it 
3) Turn off repeat for that playlist 
4) Play back the playlist in iTunes once, so it becomes the active playlist 
5) Go to Prefs > Mapping > Keynote 
6) Select "iTunes > Play" for a button of your choice

Regular use:

A) Before you start the presentation, play back the playlist in iTunes once, so it becomes the active playlist 
B) Press the button whose mapped action you changed in 6) to play back the sound once

Best regards, 
Felix Schwarz