Copy files into the following folder. Replace xxx with your computer username.
C:\Users\xxx\AppData\Roaming\nvda\addons\Unspoken-ng\globalPlugins\Unspoken\sounds

Original Theme folder: Copy files from this folder into the directory to use the default sound theme that comes pre-packaged with UnspokenNG.
VoiceOver folder: Copy the new created theme's files into the directory to start using it.
Have fun.