Here's a theme: Noise. There is the pounding cacophony of heavy metal music. It's a metaphor for the 'noise' in Ruben's life, how the touring lifestyle has in effect drowned out a part of his psyche which he needs to hear. That whole thing whereby Joe has Ruben practice sitting and meditation speaks to the converse of noise: silence.

In a way, psychologically speaking, one can look at the story this way:

Noise (Music): Inauthentic Life

Silence (Deafness): The path to an Authentic Life

It's not that black and white, and deafness is not the cure all, but it does radically redirect Ruben's existence and compel him to confront aspects of his psyche which he has managed to ignore while getting caught up in the addictive rock and roll lifestyle.

Thus, I'd say noise - and its associated dynamic silence - is one key theme.