I call out “LET ALL VOICES STOP BUT MINE!”, which is a loose English translation of the first line of a muezzin’s call to prayer in Islamic countries. I have a loud and deep voice, which may help more than the words, but it’s not something the students are expecting to here and makes them stop for a second if for no other reason than to analyze it.
Never let your voice rise (in pitch, as opposed to volume) with stress or the need to get attention. Force it deeper. Use a voice raised in volume rarely - it loses impact if you do it as a first resort rather than a last. And (I recall a teacher once telling me, in an environment where the school year started in January) never smile till Easter.
Why don’t you just say, loudly, “class, you have five seconds to stop talking. Anyone still talking after five seconds will be sent to the office.”
Enforce the rule. If they’re disobeying an order from their teacher, they’re in the wrong. Make an example out of the students who keep talking. Trust me, they’ll stop.
Er, got a cite for that? As far as I’m aware, the first line of the Adhan is “Allahu Akbar” which translates to “God is Great”.
When I was a counselor for a summer camp for 8-12 year olds, when I needed them to be quiet I would put one finger on my nose and one finger up in the air. It would take only a second for the kids to shut up and do the same. It seemed to be the universal sign for “Quiet now!”
I found myself running a college freshmen orientation session this last fall… turns out the same trick works equally well for 18-19 year olds. 
It’s not from the prayer itself, just a “Ready set go” type prelude. No idea of the nation of origin or even if it’s factual (because to be honest I don’t remember where I heard of it), but I still like it for making students hush. (I no longer make them face Mecca and pray though- frigging ACLU.)
I had two professors in law school who had little class starting routines, one who started by clapping slowly three times, one who always said “all right, let’s come together.” Putting both together might do the trick.
I once knew an MP who called his sidearm “my microphone.” People who couldn’t seem to hear what he was saying always seemed to hear him better when he used the microphone.
Actually, I used to do that… until principals started telling me to stop sending so many kids to the office.
OK, well, you could say “anyone still talking after five seconds will get an automatic F for the day.” They’ll get the message.
I believe in teachers being very strict. I raised a lot of hell in elementary, middle, and high school, but as soon as I got to college and saw all the blatant disrespect and constant talking, mumbling, listening to iPods, and other unbelievably rude behavior in class, my attitude changed dramatically.
Ah OK. I was just curious, as I’d never heard it before and googling revealed nothing.