How can I get the current sound level of the current sound output device?

The following code is pulled from an Apple SampleAVRecorder ... this particular bit of code gets the connection set from this method of the movieFileOutput class methods, gets the AVCaptureAudioChannel for each connection, and calculates the decibel power based on this. I would suggest that if you are looking for an output noise level, you can get similar information. if you are looking for something lower than this, try the Hardware Abstraction Layer (HAL) structure.

- (void)updateAudioLevels:(NSTimer *)timer
{
    NSInteger channelCount = 0;
    float decibels = 0.f;

    // Sum all of the average power levels and divide by the number of channels
    for (AVCaptureConnection *connection in [[self movieFileOutput] connections]) {
        for (AVCaptureAudioChannel *audioChannel in [connection audioChannels]) {
            decibels += [audioChannel averagePowerLevel];
            channelCount += 1;
        }
    }

    decibels /= channelCount;

    [[self audioLevelMeter] setFloatValue:(pow(10.f, 0.05f * decibels) * 20.0f)];
}