You should be able to fix the problem yourself as follows:
- Quit Dorico.
- Find the affected project in the Finder or in Windows Explorer.
- Make a copy of the affected project so that you’re not tinkering with the original.
- Change the file extension of the project from .dorico to .zip. Confirm that you want to do this when the operating system asks you whether you’re sure.
- Unzip the zip archive: on Mac, you can simply double-click it; on Windows (which I don’t have access to right now) I believe you can right-click and choose to uncompress a compressed folder.
- Go into the folder that has been created: it will contain four items, one of which is a folder called supplementary_data. Go inside this folder.
- Delete the vstaudioengine folder from inside the supplementary_data folder.
- Now go back to the folder that contains four items (e.g. score.dtn, scorelibrary.dtn, etc.), but NOT the parent folder, i.e. the folder that contains these four items.
- Select these four items (again, NOT by selecting the folder that contains these four items), right-click and choose to archive (Mac) or compress (Windows) them.
- A new zip archive called something like archive.zip will be created. Change the filename and the extension from .zip to .dorico.
- Now open up your fixed project in Dorico and you should find that it will reload all of the default sounds again for you.