If your patch doesn't sound the same as a demo, this is probably because you don’t have the full MainStage 3 sample library installed.
To install everything you need, while in MainStage 3, choose MainStage > Sound Library > Open Sound Library Manager and make sure you have everything in the graphic below downloaded.
Then restart MainStage.
If you’re tight on hard drive space, make sure you have the “Essential Sounds” and the “Legacy” content downloaded. Most of the time those two categories will resolve the issue.