The phrase "turn down" is the opposite of "turn up". To "turn down" would be to decrease the intensity of the party. And "turn down for what" means something like "don't stop the party for any reason".
OK, that makes sense in the context of "another round of shots".
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.
I had to consult with my elders to verify this, but I can now confirm that in 1950s England, "turn it up" meant the opposite: "stop what you're doing, settle down".
Pretty sure that's not the case here. To "turn down" is a common phrase (at least in the US) that is used to describe changing something by use of a control.
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.
> 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.
