It’s almost clear and obvious – turns out the loud / parent.disturbing / teen-typical music booming thru the speakers is not what Zack is listening to thru the already-donned headphones. Because instead, he is hearing. . . what, exactly?
Boom boom room
Comics I Don't Understand 
I don’t think that is the gag at all. Mom objects to the music volume, and asks him to switch to headphones, but he has already done that, and the loud volume she is hearing is actually coming from the headphones. If it’s that loud through the door, just imagine what it is doing to his eardrums.
I took it that Zack was listening to nothing at all (or pink noise, or nature sounds) on his noise-canceling earphones, while concentrating on his nerdy reading and Space posters – – and on the speakers providing a smokescreen to convince Mom that he is a normal, healthy, annoying pre-teen blasting some of their typical Boom Boom Boom music.
And BTW I think the editors were misreading the puffy lettering – it’s just Boom X 3, not a call back to the ancient trope of Boom Boom Room. This makes more sense to characterize the kind of music that would come between parent and child.
And just to continue observations of how WP handles graphics for mobile phone users, again this is much better when I turn my phone sideways. In normal (portrait) orientation, the style is compressing the Image horizontally but not vertically, giving everything a squeezed appearance.
@Kilby is 100% correct.
My primary objection to the “speakers” theory is that there isn’t anything in the drawing that looks like even one speaker, let alone two. The green thing on the left is a skateboard, and the huge black shape looks like the headboard of his bed.
Well now, that could be it, but it looks like the headphones are connected to the only speaker visible.
I agree with Kilby.
@ Maggie (7) – I assumed that the gray box was the amplifier.
Kids these days with their Special Purpose Acquisition Companies, can’t be bothered to do an IPO like normal people…
…SpacE?
Nevermind.
@ larK (9) – You can see the left edge of the next letter just behind the headboard, but the remaining space on the poster would only leave room for an “I”. Perhaps it means “Special Purpose Acquisition Company, Inc.“?
I read it slightly different: He wants to hear loud music to annoy his parents, but to protect his ears and future hearing capacity he wears headphones to dampen the volume a bit :-)
(He will still feel the bass-“Booms” through his stomach, maybe exactly how he intends to?)
@ Markus – Possible, but in that case, ear protectors (or unplugged headphones) would work just as well. The other problem is that if he is being a good little responsible music fan, why does his mother have such a look of dismay in the second panel?
@Kilby: The mother looks like that because, whatever the son is currently doing, it’s not what she meant with “use your headphones”: The rest of the family wants some quiet time.