How to detect speech start on iOS Speech API

I would recommend low-pass filtering the power signal using AVAudioRecorder and a NSTimer for callback. This way you'll be able to detect when a certain threshold is reached in the audio recorder readings, and the low-pass filtering will help mitigating the noise.

In the .h file:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h>

@interface ViewController : UIViewController{
    AVAudioRecorder *recorder;
    NSTimer *levelTimer;
    double lowPassResults;
}

- (void)levelTimerCallback:(NSTimer *)timer;
@end

In the .m file:

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // AVAudioSession already set in your code, so no need for these 2 lines.
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:nil];
    [[AVAudioSession sharedInstance] setActive:YES error:nil];

    NSURL *url = [NSURL fileURLWithPath:@"/dev/null"];

    NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
                              [NSNumber numberWithFloat: 44100.0],                 AVSampleRateKey,
                              [NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
                              [NSNumber numberWithInt: 1],                         AVNumberOfChannelsKey,
                              [NSNumber numberWithInt: AVAudioQualityMax],         AVEncoderAudioQualityKey,
                              nil];

    NSError *error;

    lowPassResults = 0;

    recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];

    if (recorder) {
        [recorder prepareToRecord];
        recorder.meteringEnabled = YES;
        [recorder record];
        levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.05 target: self selector: @selector(levelTimerCallback:) userInfo: nil repeats: YES];
    } else
        NSLog(@"%@", [error description]);
}


- (void)levelTimerCallback:(NSTimer *)timer {
    [recorder updateMeters];

    const double ALPHA = 0.05;
    double peakPowerForChannel = pow(10, (0.05 * [recorder peakPowerForChannel:0]));
    lowPassResults = ALPHA * peakPowerForChannel + (1.0 - ALPHA) * lowPassResults;  

    NSLog(@"lowPassResults: %f",lowPassResults);

    // Use here a threshold value to stablish if there is silence or speech
    if (lowPassResults < 0.1) {
        NSLog(@"Silence");
    } else if(lowPassResults > 0.5){
        NSLog(@"Speech");
    }

}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


@end

This is the code we ended up with that works.

The key thing was to installTapOnBus() and then the magic code to detect the volume,

float volume = fabsf(*buffer.floatChannelData[0]);

-(void) doActualRecording {
    NSLog(@"doActualRecording");

    @try {
    //if (!recording) {
        if (audioEngine != NULL) {
            [audioEngine stop];
            [speechTask cancel];
            AVAudioInputNode* inputNode = [audioEngine inputNode];
            [inputNode removeTapOnBus: 0];
        }

        recording = YES;
        micButton.selected = YES;

        //NSLog(@"Starting recording...   SFSpeechRecognizer Available? %d", [speechRecognizer isAvailable]);
        NSError * outError;
        //NSLog(@"AUDIO SESSION CATEGORY0: %@", [[AVAudioSession sharedInstance] category]);
        AVAudioSession* audioSession = [AVAudioSession sharedInstance];
        [audioSession setCategory: AVAudioSessionCategoryPlayAndRecord withOptions:AVAudioSessionCategoryOptionDefaultToSpeaker error:&outError];
        [audioSession setMode: AVAudioSessionModeMeasurement error:&outError];
        [audioSession setActive: true withOptions: AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation error:&outError];

        SFSpeechAudioBufferRecognitionRequest* speechRequest = [[SFSpeechAudioBufferRecognitionRequest alloc] init];
        //NSLog(@"AUDIO SESSION CATEGORY1: %@", [[AVAudioSession sharedInstance] category]);
        if (speechRequest == nil) {
            NSLog(@"Unable to create SFSpeechAudioBufferRecognitionRequest.");
            return;
        }

        speechDetectionSamples = 0;

        // This some how fixes a crash on iPhone 7
        // Seems like a bug in iOS ARC/lack of gc
        AVAudioEngine* temp = audioEngine;
        audioEngine = [[AVAudioEngine alloc] init];
        AVAudioInputNode* inputNode = [audioEngine inputNode];

        speechRequest.shouldReportPartialResults = true;

        // iOS speech does not detect end of speech, so must track silence.
        lastSpeechDetected = -1;

        speechTask = [speechRecognizer recognitionTaskWithRequest: speechRequest delegate: self];

        [inputNode installTapOnBus:0 bufferSize: 4096 format: [inputNode outputFormatForBus:0] block:^(AVAudioPCMBuffer* buffer, AVAudioTime* when) {
            @try {
                long long millis = [[NSDate date] timeIntervalSince1970] * 1000;
                if (lastSpeechDetected != -1 && ((millis - lastSpeechDetected) > 1000)) {
                    lastSpeechDetected = -1;
                    [speechTask finish];
                    return;
                }
                [speechRequest appendAudioPCMBuffer: buffer];

                //Calculate volume level
                if ([buffer floatChannelData] != nil) {
                    float volume = fabsf(*buffer.floatChannelData[0]);

                    if (volume >= speechDetectionThreshold) {
                        speechDetectionSamples++;

                        if (speechDetectionSamples >= speechDetectionSamplesNeeded) {

                            //Need to change mic button image in main thread
                            [[NSOperationQueue mainQueue] addOperationWithBlock:^ {

                                [micButton setImage: [UIImage imageNamed: @"micRecording"] forState: UIControlStateSelected];

                            }];
                        }
                    } else {
                        speechDetectionSamples = 0;
                    }
                }
            }
            @catch (NSException * e) {
                NSLog(@"Exception: %@", e);
            }
        }];

        [audioEngine prepare];
        [audioEngine startAndReturnError: &outError];
        NSLog(@"Error %@", outError);
    //}
    }
    @catch (NSException * e) {
        NSLog(@"Exception: %@", e);
    }
}