Nie Huaisang meant for it to be a surprise. Learning the Song of Clarity for his brother, so that he wouldn't need to feel bad about his sworn brothers travelling all the way to Qinghe for him - it would be the perfect birthday gift. Obviously he can't just ask someone to teach him, since they would just tell his brother about it, but that's fine; Nie Huaisang knows of a place where he can sit and listen in secret - he'll just pick it up that way. Except something must have gone wrong, because he's started coughing up blood...