Something in which to jot down music?
I believe the answer is:
notebook
'music?' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for notebook that I've seen before include "Small compact portable computer" , "Jotter - portable computer" , "Jotter's accessory?" , "Small volume for brief records" , "Bigger than diary, smaller than laptop" .)