According to my impeccable knowledge of manga and anime Japanese sex sounds can be heard approximately in a reach of 23.7m in all directions irrespective of whether there's any wall between source and listener, and irrespective of what type of wall it is.
Therefore it would have been pointless trying to stifle her sounds either way, unless she was prepared to go to a love hotel just to masturbate (which presumably all Japanese teens have to do when figuring out their sexuality if they don't want to be overheard by their parents).