Really, singing wasn't Riku's talent. It really was story telling, and his poetry. But he if could occasionally carry a small tune for his Beloved, and it pleased him, it was good enough.
"I remember that," Riku murmured, "You taught her to sing in Trabian." A secret message for him that his guards couldn't understand. It had given him comfort, and hope.
Yes, not going to tell his brother about that episode in their relationship. He just wouldn't understand.
Corin recognized the song that the Prince began playing (or.... thought he did? The middle part there was a little different than he recalled, but perhaps it was a nobility thing. Either way, the Prince clearly had talent. More, it seemed to calm his little brother, as Riku abandoned his tea and just kind of had his chin in his hands and listened happily to the Prince play, his eyes soft and vaguely distant.
"You play beautifully," Corin said once the Prince finished, and Riku just reached out his hands happily for his Beloved.