If it's noisy, just use that to your advantage. Position yourself next to an attractive girl you want to approach and after a few seconds turn to her and say:
You: "What?" (loudly)
Her: "What?"
You: "Sorry, I thought you said something"
And then you go into any opener of your choosing. I've used this to great success. If not just make small talk between the different bands that are playing: "What band is playing next?" "Who were those guys (who just played)? they were really good!" Once you've already established rapport my favorites for making post-show plans "Where's a good bar to drink after this? I hate buying drinks at shows and getting them spilled all over me."
~Antithesis