Welcome,
So far, this kind of error has always been due to the wrong settings on the “Add” import dialog such as the ones referenced in this similar issue with acoustic model resources being added or not added:
https://www.politepix.com/openears/support/#Q_My_app_crashes_when_listening_starts
Basically, the voice resources were not successfully added to your more complex project in the expected location in the bundle, but they were successfully added to the brand-new project in the expected location in the bundle. In the more complex project they are probably in a subfolder inside the app bundle where NeatSpeech can’t find them because they are expected to be at root level. This difference is a result of the setting mentioned here:
Make absolutely sure that in the add dialog “Create groups for any added folders” is selected and NOT “Create folder references for any added folders”