It could be that you have the favorites-only selected. Just tap the heart in the middle of the screen.
The purpose of this feature is to let you mark your loved sounds as favourites, and then with favourites-only selected you can access them really quickly (even when half asleep)
This may have occurred because there wasn't enough space on your device. Please go to the app's sounds store and tap the cloud icon next to the sounds that disappeared or click the Restore button at the bottom.