This is the book I used.
This is a book I used.
This is the music I listened to.
This is a music I listened to.
1 and 2 are implying that the book and the music were mentioned before at least once, so the listener already knows some part of the stories.
Whereas, 2 is implying that the listener is hearing about them having studied that book for the first time.
I want to make 4 the same as 2, but I can't use determiner 'a' in front of non-countable noun. What should I do? What am I missing?