http://stephencelis.com/2009/03/02/now-i-just-need-an-audience.html

gistfile1.m

#import "SCListener.h" // Remember to link to AudioToolbox.framework.

// Start listening.
[[SCListener sharedListener] listen];

// Retrieve the average power.
[[SCListener sharedListener] averagePower];

// Retrieve the peak power.
[[SCListener sharedListener] peakPower];

// Hmm...we're using this guy a lot...
SCListener *listener = [SCListener sharedListener];

// We can temporarily stop returning levels
[listener pause];
[listener listen]; // Quick.

// Or free up resources when we're not listening for awhile.
[listener stop];
[listener listen]; // Slower.

// Advanced!:
//
// If you're using the average and the peak, fetch both at once.
if (![listener isListening]) // If listener has paused or stopped...
  return;                    // ...bail.

AudioQueueLevelMeterState *levels = [listener levels];
Float32 peak = levels[0].mPeakPower;
Float32 average = levels[0].mAveragePower;