You could try RCDefaultapp from
http://www.rubicode.com/Software/RCDefaultApp/
Freeware and apparently still works fine with Leopard.
From site..."RCDefaultApp is a Mac OS X 10.2 or higher preference pane that allows a user to set the default application used for various URL schemes, file extensions, file types, MIME types, and Uniform Type Identifiers (or UTIs; MacOS 10.4 only). MacOS X uses the extension and file type settings to choose the application when opening a file in Finder, while Safari and other applications use the URL and MIME type settings at other times for content not related to a file (such as an unknown URL protocol, or a media stream)."
Setting should be under MIME types - scroll down for audio and select application you want to use for each audio type.
HTH
Steve