This is a wonderful trick, and a must-know for anyone who hangs out in bars or at concerts, racetracks, the New York Stock Exchange or family reunions. It's also a great way to introduce yourself to new people and is a perfectly acceptable way of picking someone up (if you're not at the aforementioned reunion) assuming they don't run away at what you're about to propose: basically, you're going to ask them if it's alright for you to stick your finger in their ear.
This is how it works: go up to the person you want to talk to. Gesture to them that they should turn their head to one side. When they do, press your index finger gently against their tragus, the flap of cartilage that covers their auditory canal. I know what I said before; that was a joke. Don't actually stick your finger in their ear. There's a good shot they'll either slap you, scream or seriously injure themselves by jerking their head in a dangerous direction.
Once you've closed off their ear canal, lean in and speak in a normal voice, focusing your words on the nail of your finger. Due to the magic of bone conduction they should be able to hear you.
The one thing you need to be careful of is piercings; some people have studs through that particular bit of cartilage and pressing your finger against it can hurt. All is not lost, however; simply find a way to turn their head the other way, the smoother the better.