What the hell is boom sound and why does it need an icon?
I would have guessed it was location services/GPS as it kind of looks like that on other systems.
Boom sound is an HTC term for their dual front stereo speakers. Doesn't really mean anything, kind of like Apple coining the term 'Retina Display'. Doesn't mean a thing.
And no idea why in the hell HTC needs a status bar icon for it?
BoomSound is an equalizer, it can't be disabled using the internal speakers. If you plug in headphones and play music by default you'll see the BoomSound icon on top but if you disable BoomSound in settings with headphones plugged in, the icon will go away too. You'll notice the sound signature change as well.