Sound files in System/Library/Sounds gone with macOS Sierra upgrade?
I have upgraded to macOS Sierra (10.12) from OS X El Capitan (10.11) as well.

macOS 10.12: In my /System/Library/Sounds/ directory there are 14 AIFF sound files with a time stamp of Aug 20 2016.

In OS X El Capitan there are the same 14 AIFF sound files in the same directory. Their timestamp is Aug 26 2015.

I have confirmed the same files for three other Macs that were upgraded from 10.11 to 10.12.



To answer your question:
Yes, you should be able to copy the AIFF files to your /System/Library/Sounds/ directory from another installation.

Please keep in mind that you need to copy them using sudo as this is a system folder.