First off i have Flash CS4 professional and flash player 10

For some reason I cant get my flash buttons to play sound anymore. I had a quick go at making a flash soundboard a while ago and it worked, I have now gone back and tried to make another one but I cant get the buttons to play sound.

Here is what I do:

Open a new flash file (I have tried both 2.0 and 3.0)
Draw a simple shape
Convert it to a button symbol
Go into the symbol
Add a key frame to down
Import my sound (have tried both mp3 and wav)
Make sure down is still selected then drag the sound from the library to the button (when I do this I can see the sound file show up in the down key frame)

Then when I test it and click on the button nothing happens. I have tried adding a colour change to the down key frame which does change colour when I click on the button, so I know the button is responding to my click, but it will not play any sounds I add to the keyframe. I have even tried reinstalling flash, but that didnt help either. I have used another computer with flash on it and I am able to get the buttons to work.

Any ideas on why I cant get sound to play through my buttons?