right，but I personally think it’s more than that 1 你曾听过这样温柔的弹奏. you (must) have heard such tender playing like this before. 2 你曾听过这样温柔的弹奏? you have heard such tender playing like this before? 3 你可 (=可否) 曾经听过这样温柔的弹奏? Have you ever heard...? 4 你可 (=可能) 曾经听过这样温柔的弹奏. you may have heard such tender playing like this before... 5 你可 (=可是) 曾经听过这样温柔的弹奏. you must have heard.... this one here is for emphasis it fits specific literary thing,as for mood or bringing rythem to a poem， lyrics and tv show dialogue.well it's quite Cliche. 你可知道那是来自远方的心声。 你可懂汝父用心良苦。 I made some complicated conversation here. Zhang -你可知道那是什么？ Li -我可不知道。 Wang -我可知道，那是一个盒子。 Li -那可不是个盒子。 Zhang -可不是呢。 wang -可如果那不是一个盒子又是什么？ Li -你可别卖关子了，快告诉我们吧。 Zhang -可是我也不知道呀。

You are right. 可 here just emphasizes 曾, without specific meaning. Other examples: 他可不是我的男朋友！ 他腹黑极了，你可要小心！

Hi, Beth, I think your translation is perfectly fine. "可" is used to emphasise “曾”. However, it might also suggest a possibility, but I might be wrong. Is it possible that you have heard this before? means 'you must haven't heard this before.' 你可曾听过这样温柔的弹奏. Actually means such tender playing is the best tune I have ever heard. I hope it helps. Joyce

用在问句里,加强语气 [used to form an interrogative sentence]。如:你可听过他杀过人? From Baidu