You can set your package type to per-user from the install parameters page. The extensions will then be per user. Just make sure all files are installed in per-user location, like in the "Application Data" folder.
Thanks for you response.
This doesn't work as the installer tries to write to a protected registry key for the file association. The error message is:
"Cannot write to:
\SOFTWARE\Digitope\AudioEditor\Capabilites\FileAssociations"
I think I need to change the file association capabilities path in Advanced Installer, which is SOFTWARE, but I don't know the right key.