Do you see Loki as the type to willingly become intoxicated in public?
Not taking his sense of dignity into account, he is the smallest guy there. Thor and all his buddies are most likely there, also drinking. Him getting drunk around them is like asking them to publicly embarrass him. Like true brothers. xD Just my thoughts.
Oh dear gods, Thor's "ridiculously-happy-and-possibly-quite-drunk-too" expression is hilarious when combined with karaoke! Poor Loki looks so uncomfortable in this scenario, Thor probably dragged him into this XD
I'm guessing Thor dragged Loki here, had much too much to drink, and is now singingly loudly and off-key as Loki wonders whether he could slip out and head for home without Thor noticing.... *grins* So cute!