iOS イヤホンジャックに機器を接続した状態で、本体の内蔵スピーカーから音を鳴らす ref: http://qiita.com/t_oginogin/items/c0034a3d830d28ec18e5

SampleViewController.h

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

@interface SampleViewController : UIViewController <AVAudioPlayerDelegate>
@end

SampleViewController.m

#import "SampleViewController.h"
#import <AVFoundation/AVAudioSession.h>
#import <AudioToolbox/AudioServices.h>

@interface SampleViewController ()
{
    AVAudioPlayer *player;
}
@end

@implementation SampleViewController
- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    // overrideOutputAudioPort:でAVAudioSessionPortOverrideSpeakerを設定するための準備
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:nil];

    // AudioSessionを有効化
    [[AVAudioSession sharedInstance] setActive:YES error:nil];

    // 電話が鳴るなどAudioSessionの割り込み発生時の処理を登録
    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
    [center addObserver:self selector:@selector(sessionDidInterrupt:) name:AVAudioSessionInterruptionNotification object:nil];

    // 鳴らす音の準備
    NSString* path = path = [[NSBundle mainBundle] pathForResource:@"test" ofType:@"aiff"];
    NSURL* url = [NSURL fileURLWithPath:path];
    player = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:nil];
    player.delegate = self;
}

// AudioSessionの割り込み発生時の処理
- (void)sessionDidInterrupt:(NSNotification *)notification
{
    switch ([notification.userInfo[AVAudioSessionInterruptionTypeKey] intValue]) {
        case AVAudioSessionInterruptionTypeBegan:
            [player stop];
            break;
        case AVAudioSessionInterruptionTypeEnded:
        default:
            [[AVAudioSession sharedInstance] setActive:YES error:nil];
            break;
    }
}

// サウンド再生が終わったときの処理
- (void)audioPlayerDidFinishPlaying:(AVAudioPlayer *)player successfully:(BOOL)flag
{
    // 内蔵スピーカーへの強制出力を止める
    [[AVAudioSession sharedInstance] overrideOutputAudioPort:AVAudioSessionPortOverrideNone error:nil];
}

- (IBAction)sound:(id)sender {
    // 内蔵スピーカーへ強制出力
    [[AVAudioSession sharedInstance] overrideOutputAudioPort:AVAudioSessionPortOverrideSpeaker error:nil];
    [player play];
}

@end