I guess no one with loud music considers themself an asshole, so this should be actually giving the information on exactly what you mention.
https://www.youtube.com/watch?v=HMUDVMiITOU
https://www.youtube.com/watch?v=X_IWlPHMziU
Other cultures seem to feel more entitled, thinking that THEIR music could not possibly bother anyone. I've certainly heard people blast Wagner or Orff at high volumes.
As described at Wiktionary [0] - it's an idiomatic way of saying that you're going to lower the volume through use of a control to do that. The context that was used has nothing to do with party.
[0] https://en.wiktionary.org/wiki/turn_down
EDIT: My bad, thought it was in response to...
> I guess it will mostly reflect the musical taste of assholes who turn their music up loud. Hmm, but maybe all culture works like that.
> At its core, turn down for what is a phrase used to promote having a good time. The phrase itself implies that there is no reason to turn down and stop partying.
But in my experience, party intensity and music volume are generally correlated, so you would probably turn down the former by turning down the latter.
A collogue:
Someone who sees their role on the team as to annoys others.
Mine is firing directly upwards. I'm not trying to knock birds out of the sky.
My subs, while currently pointing backwards, would have been better firing upwards for no other reason than the manufacturer (Audiofrog) doesn't recommend grills. As it is, I have to be careful what I place in my trunk to avoid punching a hole in the cone.