Hey Dom,
It's actually not that hard, but is buried deep where you wouldn't find it in a long time (in typical MS fashion).
Insert the sound file in whatever slide you want it to start playing, as usual.
Then go to the 'Slide Show' menu and select 'Custom Animation'
Click on the sound file down the left hand side, then click on 'Effect Options'. Click on the 'Media Options' tab, and then click the radio button 'Stop playing after x number of slides', and set the number to 1000 or something.
Done! This is in PowerPoint 2004, although I think the procedure is the same in PowerPoint X. Not sure about '98.