マナーモードでも音を出したい場合は、AudioSessionSetProperty() でカテゴリ指定をすると変更できる。

- (void)loadAudioMode:(BOOL)isLiveMode {
    AudioSessionInitialize(NULL, NULL, NULL, NULL);
    UInt32 sessionCategory;
    if (isLiveMode) {
        sessionCategory = kAudioSessionCategory_LiveAudio;
    } else {
        sessionCategory = kAudioSessionCategory_SoloAmbientSound;
    }
    AudioSessionSetProperty(kAudioSessionProperty_AudioCategory,
                            sizeof(sessionCategory), 
                            &sessionCategory);
    AudioSessionSetActive(YES);
}

Audio Sessionカテゴリ

AudioSirvices クラスに定義してあり、iPhone Core Audio プログラミングの書籍に記載されているも定義が多いです（SDKのバージョンがあがったから？）。

enum { // typedef UInt32 AudioSessionPropertyID
    kAudioSessionProperty_PreferredHardwareSampleRate           = 'hwsr',   // Float64          (get/set)
    kAudioSessionProperty_PreferredHardwareIOBufferDuration     = 'iobd',   // Float32          (get/set)
    kAudioSessionProperty_AudioCategory                         = 'acat',   // UInt32           (get/set)
    kAudioSessionProperty_AudioRoute                            = 'rout',   // CFStringRef      (get only)
    kAudioSessionProperty_AudioRouteChange                      = 'roch',   // CFDictionaryRef  (property listener)
    kAudioSessionProperty_CurrentHardwareSampleRate             = 'chsr',   // Float64          (get only)
    kAudioSessionProperty_CurrentHardwareInputNumberChannels    = 'chic',   // UInt32           (get only)
    kAudioSessionProperty_CurrentHardwareOutputNumberChannels   = 'choc',   // UInt32           (get only)
    kAudioSessionProperty_CurrentHardwareOutputVolume           = 'chov',   // Float32          (get only/property listener)
    kAudioSessionProperty_CurrentHardwareInputLatency           = 'cilt',   // Float32          (get only)
    kAudioSessionProperty_CurrentHardwareOutputLatency          = 'colt',   // Float32          (get only)
    kAudioSessionProperty_CurrentHardwareIOBufferDuration       = 'chbd',   // Float32          (get only)
    kAudioSessionProperty_OtherAudioIsPlaying                   = 'othr',   // UInt32           (get only)
    kAudioSessionProperty_OverrideAudioRoute                    = 'ovrd',   // UInt32           (set only)
    kAudioSessionProperty_AudioInputAvailable                   = 'aiav',   // UInt32           (get only/property listener)
    kAudioSessionProperty_ServerDied                            = 'died',   // UInt32           (property listener)
    kAudioSessionProperty_OtherMixableAudioShouldDuck           = 'duck',   // UInt32           (get/set)
    kAudioSessionProperty_OverrideCategoryMixWithOthers         = 'cmix',   // UInt32           (get, some set)
    kAudioSessionProperty_OverrideCategoryDefaultToSpeaker      = 'cspk',   // UInt32           (get, some set)
    kAudioSessionProperty_OverrideCategoryEnableBluetoothInput  = 'cblu',   // UInt32           (get, some set)
    kAudioSessionProperty_InterruptionType                      = 'type',   // UInt32           (get only)
};