《九尾狐传1938》中柳红酒的主题曲OST是由GRASS演唱的《Keep Silence》正在展开超乎想象的第二幕的tvN《九尾狐传1938》，给人留下深刻回响的最后一首OST发行。《Keep Silence》表达了柳红酒（金素妍 饰）虽然深爱着李砚（李栋旭 饰），但却无法拥有的哀切心情的歌曲。预计会帮助观众投入感情，带来巨大的感动。歌曲以抒情的钢琴与强烈的电吉他和合成器声音作强烈的比较，加上担任演唱的GRASS细腻的主唱力，进一步提高了对剧情的投入度。GRASS以旅游创作者兼歌手的身份运营的视频频台「It's me GRASS 그래쓰」获得大众的喜爱，继《浪漫速成班》之后，参与《九尾狐传1938》，坚持不懈地开展着音乐活动。[Credit]Lyrics by B&NAz(153/Joombas)Composed by Quokka(Stupid Squad), 오형석(153/Joombas), RYVNG(153/Joombas)Arranged by 오형석(153/Joombas), RYVNG(153/Joombas)Piano by 오형석Guitar by 오형석Drum by RYVNGProgramming by 오형석 Background Vocal by Quokka, 강다나, 그래쓰Recorded by 엄세현 @GCA StudioMixed by 고현정 @koko SoundMastered by 박준 도정회 @SoundMAXOST Producer 김정하 송예진 김민호