How can I say "May I have your attention, please?" in fewer words

You could simply shorten it to, "Attention, please".

Shouting silencio does the trick. It's not as informal as shouting oye and it works phonetically because you can emphasise the o at the end. Shouting silence doesn't work as well, because it may not be heard when the crowd is noisy.

According to Urban Dictionary's entry of silencio:

The Spanish Word for silence. Used to quiet various size groups of people in a fast effective manner. From Chapelle's Show Season 2, the "Racial Draft 2004."

I looked up that clip from the Chappele's show, but it's not used properly because it's not shouted and the emphasis isn't on the end.

I think the trick to a good silencio shout is taking your time with the first i (about 1 second), this draws attention, and emphasising the o at the end to show the audience you're serious.

Attribution: "Silencio." Urban Dictionary. Accessed April 18, 2018. https://www.urbandictionary.com/define.php?term=Silencio.

You can simply say, "Excuse me". the expected response is for the group to turn to look at you or say something like "Yes?" or "Hmm?", at which point you have their attention.

excuse me 1 Used as a polite apology in various contexts, such as when attempting to get someone's attention, asking someone to move so that one may pass, or interrupting a speaker. - ODO

It captures the essence of a request for permission to speak (further).