By default, the OS9 version of iTunes stores stuff in:
Mac HD/Documents/iTunes/iTunes Music/
However, if the songs were left in their original locations (not copied into iTunes) then that location will be empty. But music files take up a lot of space. So ...
Open the OS9 hard drive in List view. Set the window to calculate all sizes, then sort by Size. Click disclosure triangles as needed, follow the larger sizes until you find the audio files.