Sound alarm with Smart Bluetooth
You may not need to develop anything actually. There are devices on the market which already provide the functionality you describe. Have a look at the following two for example:


The Treasure Tag by Nokia, now Microsoft

And despite promoting mainly Windows Phones, there's an iPhone App for it

The StickNFind tags


However if you are dead set on developing your own hardware, the common way is to put attach a PWM output to a transistor/fet which drives a buzzer or alternatively there are buzzers which directly generate a noise when you just apply a voltage. 

None of the commonly used Bluetooth 4.0 single mode chips are capable of feeding enough current directly to drive a buzzer directly, but on the plus side they pretty much all have PWM outputs.