Click the file and click "Get info" (in OSX you control-click it or press Command-I. I'm not too sure 'bout OS9). Click "Open with" and set iTunes as the default app. Then click "change all".

Now, this may or may not be accurate because I'm telling you how to do it on OSX. Try it though, you never know.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-