Four ways to gain the attention of the class.

• When you're within hearing range of a few children, say in a normal-level voice, "if you can hear me, clap once, " "if you can hear me, clap twice," etc... usually I have the whole class's attention by the 4th time or so... sometimes I'll throw in a wierd direction like "if you can hear me, touch your nose..." the kids enjoy it and it works pretty well.
• I've also seen teachers who will do a clapping pattern and the kids copy... again, this gets them paying attention quickly... something with clapping seems to get kid's attention fast.
• One of the teachers I student taught with had a little bell she'd ring to get their attention. Simple solution.
• Another one I've seen is simply saying "1-2-3 Eyes on me." You have to be a little louder, but it usually works... or simply counting "1, 2, 3, etc" really slowly, but loud enough that they can hear you.