Isn it possible to undo actions in the repair center?
Yes it is! The reason why the “Undo” menu item can be grayed out is just because it applies to the frontmost window, which should be the sound window. So, just select the sound window and you can undo as usual. You can also use the handy ‘z’ key shortcut (no command, just ‘z’) to undo and ‘Z’ to redo.