So, I've noticed that there are piano's in the game that Max can play, and, although they aren't collectables, Max's ability (or inability) to play them appear to reflect on his current emotional state or sense of purpose. In Chapter I, right before he dives across the roof to save Fabiana, he attempts to play the game's theme and fails. Near the end of the boat sequence in the Panama flashback, he attempts to play only a few notes (harmonized to the badass backing track for that stage). Finally, Max finds an out-of-tune piano in the run-down hotel in Chapter XII and successfully plays the main theme.
Did you guys notice these on your playthrough, too? Did I miss any? I thought they were nice little easter eggs that added a bit to Max's character arc.