Hello Steve!
I have tested the installer with Sibelius 6 and Windows 7 here and it worked fine. Maybe you are looking at the wrong folder or you have older versions of the sound sets installed in a different folder. Sibelius looks at several places for sound sets and if it finds sound sets with the same name at different places, it will only display one version of them.
For Windows 7, Sibelius 6 the sound sets get installed to the following folder:
Users\your username\AppData\Roaming\Sibelius Software\Sibelius 6\Sounds
Please check if the sound set files have been installed to this folder.
It may also be necessary to remove sound set files with the same names manually, if you have them located in different Sibelius 6/Sounds folders.
Best regards,
Andi
Vienna Symphonic Library