ex8404 is right, along with the 5 you begin with, you can gain 5 more (Specifically songs 11-15) by collecting "Sheet Music" in the Cannons of the Deep mobile phone game. Fortunately we have found a way to access the mobile game without the need of a phone with internet, and it can be found by following the link below, or finding the forum listing below in the main forum chart:
POTCO Mobile: Cannons Of The Deep
POTCO's Cellphone game - Tips, Tactics, Discussion, etc.
There are still plenty of other songs that remain undiscovered as of now, and there is no viable way to reach them at this point.
Also, for those who haven't noticed yet, Disney added yet another new easter egg for the holiday season this year. You will find two holiday tavern pieces at the bottom of the listings while the Winter Holiday is in progress.