That's interesting and if that's the case we will need to fix that. I want to make sure of the sequence of events here: You go to Tools->Options->Audio->Load then you are selecting the rtst_vpk that is the larger file size?
(The extension is the same from version 9, but the file is a different format. The idea behind that was for easier backwards-compatibility, since Beta 10 will still understand the old format, providing the original sound files are there)
If that isn't working, open up the voice pack configuration options and choose File->New. That should reload the original settings contained in our Achievements component. Then test to make sure the default sounds play, and try reloading the custom voice pack with the steps described above.