"Listener" (also on AO3):

Nardole stood outside the Vault. He didn't have a glass pressed to the door, as such, but he did have his bionic ear turned to the max. The Doctor had been inside for way too long.

Were they talking? Fighting? ... Did he want to know? He couldn't hear anything.

Finally a sound: she was playing scales, over and over, thumping the keys. (What was he doing?)

A guitar riff harmonized with the obsessive scales. She changed keys, clashing, but the Doctor followed. They chased each other around the circle of fifths until Nardole almost thought they were actually making music.