Help > FAQ > Backup & restore sound list
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To restore a backup select File > Restore sound list from the menu.
Soundpad stores a working copy of the currently loaded sound list periodically and on application exit at %Appdata%\Leppsoft (equal to C:\Users\Username\AppData\Roaming\Leppsoft\soundlist.spl).
If you accidentally clicked on a table column and sorted your list, you can select Edit > Undo from the menu to get your old sort order back. But, if you already closed Soundpad then the working copy was updated and the undo history is gone.
If you want to prevent sorting of the list at all, so it doesn't happen to you again, you can do so by checking File > Preferences > Interface > Disable sorting in sound list.
More info: Managing sound lists.