In Netscape under options, general preferences, go to helpers and make sure there is an entry for audio/aiff, and perhaps also for audio/x-aiff, to be handled by the "LiveAudio" plugin; this plugin should come with any standard distribution of the Netscape browser.
With Internet Explorer, edit preferences, and choose file helpers. Make sure there is an entry for AIFF files with a .aiff extension to be handled by Internet Explorer.