Could be that the conversion settings have a lower sample rate than what they need. Just to be sure, does this also happen when you listen to the ORIGINAL sound instance on Wwise? If it doesn't then you need to adjust the conversion settings. To do so, go to the Conversions Tab on the Property Editor of the parent of these objetcs, and then either use a custom or a new ShareSet. From "Edit..." you can adjust all of these things, such as sample rate, format, compression rate, etc.
This shouldn't happen because of using a free license.
Let me know if you could solve it, so in case you couldn't, we can think of something else.
