Drinking a little just puts me in a good mood and makes me a little more comfortable in otherwise daunting social situations.
Drinking a bit more and I start to feel like singing a lot. Usually whatever obscure metal I had most recently been listening or felt like listening to. Occasionally this mood goes well with karaoke, but if I drink too much then I sing like crap. If I don't drink enough, then I'm nervous. Haven't done it with the right balance yet but I'm working on it. [note: may not help that I never seem to let myself sing anything easy, I've done a lot of painful karaoke Queensryche performances]
Also at a certain point, I develop a tolerance for music I otherwise despise. That is, suddenly I don't mind whatever annoying hip-hop or dance music is being played at the party or bar or wherever I am, which is helpful. To be standing there with this "I hate your music" look doesn't really help in getting along with people at those kind of parties. I'm pretty tolerant of other peoples' musical tastes, but after a few drinks I somehow start to like everything (and don't mind dancing a bit, very hard to get me to do othewise). Except country. I haven't drank enough yet to start liking country.
Beer seems to make me a little drowsy, especially if I'm already tired, which is why I prefer mixed drinks and Redbull and Vodka is really great 'cause it makes sure I don't get tired. Of course, I've noticed that I can get a lot more crazy on enough caffeine (or similar chemicals in energy drinks and such) than alcohol. Someday I aspire to set a new trend among rockstars by going on caffeine induced rampages and get a VH1 Behind the Music for my band where they mostly talk about "Matt's increasingly debilitating caffeine habit."
"By the end of our 2008 tour, I needed 3 redbulls just to get on stage, and after the show I would down about two liters of mountain dew, drop my pants around my ankles and run through the backstage area shouting 'I can teleport! The tacos can't hide from me now!'"